Solidity version 0.7.0

contracts/FxChild.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 // IStateReceiver represents interface to receive state
 interface IStateReceiver {

contracts/FxRoot.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 
 interface IStateSender {

contracts/examples/erc1155-transfer/FxERC1155ChildTunnel.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import {IFxERC1155} from "../../tokens/IFxERC1155.sol";
 import {ERC1155Holder} from "../../lib/ERC1155Holder.sol" ;

contracts/examples/erc1155-transfer/FxERC1155RootTunnel.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import {ERC1155} from "../../lib/ERC1155.sol";
 import {ERC1155Holder} from "../../lib/ERC1155Holder.sol" ;

contracts/examples/erc20-transfer/FxERC20ChildTunnel.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import { FxBaseChildTunnel } from '../../tunnel/FxBaseChildTunnel.sol';
 import { Create2 } from '../../lib/Create2.sol';

contracts/examples/erc20-transfer/FxERC20RootTunnel.sol

@@ -1,10 +1,10 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import { ERC20 } from "../../lib/ERC20.sol";
 import { Create2 } from "../../lib/Create2.sol";
 import { FxBaseRootTunnel } from "../../tunnel/FxBaseRootTunnel.sol";
-import {SafeERC20,IERC20} from "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";
+import {SafeERC20,IERC20} from "../../lib/SafeERC20.sol";
 
 /**
  * @title FxERC20RootTunnel

contracts/examples/erc721-transfer/FxERC721ChildTunnel.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import { FxBaseChildTunnel } from '../../tunnel/FxBaseChildTunnel.sol';
 import { Create2 } from '../../lib/Create2.sol';

contracts/examples/erc721-transfer/FxERC721RootTunnel.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import { ERC721 } from "../../lib/ERC721.sol";
 import { Create2 } from "../../lib/Create2.sol";

contracts/examples/mintable-erc20-transfer/FxMintableERC20ChildTunnel.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import { FxBaseChildTunnel } from '../../tunnel/FxBaseChildTunnel.sol';
 import { Create2 } from '../../lib/Create2.sol';

contracts/examples/mintable-erc20-transfer/FxMintableERC20RootTunnel.sol

@@ -1,11 +1,11 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import { Create2 } from "../../lib/Create2.sol";
 import { SafeMath } from "../../lib/SafeMath.sol";
 import { FxERC20 } from "../../tokens/FxERC20.sol";
 import { FxBaseRootTunnel } from "../../tunnel/FxBaseRootTunnel.sol";
-import {SafeERC20,IERC20} from "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";
+import {SafeERC20,IERC20} from "../../lib/SafeERC20.sol";
 
 
 /** 

contracts/examples/state-transfer/FxStateChildTunnel.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import { FxBaseChildTunnel } from '../../tunnel/FxBaseChildTunnel.sol';
 

contracts/examples/state-transfer/FxStateRootTunnel.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import { FxBaseRootTunnel } from '../../tunnel/FxBaseRootTunnel.sol';
 

contracts/lib/Address.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 /**
  * @dev Collection of functions related to the address type

contracts/lib/Context.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 /*
  * @dev Provides information about the current execution context, including the

contracts/lib/Create2.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 
 // Create2 adds common methods for minimal proxy with create2

contracts/lib/ERC1155.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import "./IERC1155.sol";
 import "./IERC1155Receiver.sol";

contracts/lib/ERC1155Holder.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import "./ERC1155Receiver.sol";
 

contracts/lib/ERC1155Receiver.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import "./IERC1155Receiver.sol";
 import "./ERC165.sol";

contracts/lib/ERC165.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import "./IERC165.sol";
 

contracts/lib/ERC20.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import { IERC20 } from './IERC20.sol';
 import { SafeMath } from './SafeMath.sol';

contracts/lib/ERC721.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import "./IERC721.sol";
 import "./IERC721Receiver.sol";

contracts/lib/IERC1155.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import "./IERC165.sol";
 

contracts/lib/IERC1155MetadataURI.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import "./IERC1155.sol";
 

contracts/lib/IERC1155Receiver.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import "./IERC165.sol";
 

contracts/lib/IERC165.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 /**
  * @dev Interface of the ERC165 standard, as defined in the

contracts/lib/IERC20.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 /**
  * @dev Interface of the ERC20 standard as defined in the EIP.

contracts/lib/IERC721.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import "./IERC165.sol";
 

contracts/lib/IERC721Metadata.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import "./IERC721.sol";
 

contracts/lib/IERC721Receiver.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 /**
  * @title ERC721 token receiver interface

contracts/lib/Merkle.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 library Merkle {
     function checkMembership(

contracts/lib/MerklePatriciaProof.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import {RLPReader} from "./RLPReader.sol";
 

contracts/lib/Ownable.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 /**
  * @dev Contract module which provides a basic access control mechanism, where

contracts/lib/RLPReader.sol

@@ -2,7 +2,7 @@
 * @author Hamdi Allam hamdi.allam97@gmail.com
 * Please reach out with any questions or concerns
 */
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 library RLPReader {
     uint8 constant STRING_SHORT_START = 0x80;

contracts/lib/SafeERC20.sol

@@ -0,0 +1,77 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity >=0.7.0<0.8.0;
+
+import "./IERC20.sol";
+import "./SafeMath.sol";
+import "./Address.sol";
+
+/**
+ * @title SafeERC20
+ * @dev Wrappers around ERC20 operations that throw on failure (when the token
+ * contract returns false). Tokens that return no value (and instead revert or
+ * throw on failure) are also supported, non-reverting calls are assumed to be
+ * successful.
+ * To use this library you can add a `using SafeERC20 for ERC20;` statement to your contract,
+ * which allows you to call the safe operations as `token.safeTransfer(...)`, etc.
+ */
+library SafeERC20 {
+    using SafeMath for uint256;
+    using Address for address;
+
+    function safeTransfer(IERC20 token, address to, uint256 value) internal {
+        _callOptionalReturn(token, abi.encodeWithSelector(token.transfer.selector, to, value));
+    }
+
+    function safeTransferFrom(IERC20 token, address from, address to, uint256 value) internal {
+        _callOptionalReturn(token, abi.encodeWithSelector(token.transferFrom.selector, from, to, value));
+    }
+
+    function safeApprove(IERC20 token, address spender, uint256 value) internal {
+        // safeApprove should only be called when setting an initial allowance,
+        // or when resetting it to zero. To increase and decrease it, use
+        // 'safeIncreaseAllowance' and 'safeDecreaseAllowance'
+        // solhint-disable-next-line max-line-length
+        require((value == 0) || (token.allowance(address(this), spender) == 0),
+            "SafeERC20: approve from non-zero to non-zero allowance"
+        );
+        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, value));
+    }
+
+    function safeIncreaseAllowance(IERC20 token, address spender, uint256 value) internal {
+        uint256 newAllowance = token.allowance(address(this), spender).add(value);
+        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, newAllowance));
+    }
+
+    function safeDecreaseAllowance(IERC20 token, address spender, uint256 value) internal {
+        uint256 newAllowance = token.allowance(address(this), spender).sub(value, "SafeERC20: decreased allowance below zero");
+        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, newAllowance));
+    }
+
+    /**
+     * @dev Imitates a Solidity high-level call (i.e. a regular function call to a contract), relaxing the requirement
+     * on the return value: the return value is optional (but if data is returned, it must not be false).
+     * @param token The token targeted by the call.
+     * @param data The call data (encoded using abi.encode or one of its variants).
+     */
+    function _callOptionalReturn(IERC20 token, bytes memory data) private {
+        // We need to perform a low level call here, to bypass Solidity's return data size checking mechanism, since
+        // we're implementing it ourselves.
+
+        // A Solidity high level call has three parts:
+        //  1. The target address is checked to verify it contains contract code
+        //  2. The call itself is made, and success asserted
+        //  3. The return value is decoded, which in turn checks the size of the returned data.
+        // solhint-disable-next-line max-line-length
+        require(address(token).isContract(), "SafeERC20: call to non-contract");
+
+        // solhint-disable-next-line avoid-low-level-calls
+        (bool success, bytes memory returndata) = address(token).call(data);
+        require(success, "SafeERC20: low-level call failed");
+
+        if (returndata.length > 0) { // Return data is optional
+            // solhint-disable-next-line max-line-length
+            require(abi.decode(returndata, (bool)), "SafeERC20: ERC20 operation did not succeed");
+        }
+    }
+}

contracts/lib/SafeMath.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 /**
  * @dev Wrappers over Solidity's arithmetic operations with added overflow

contracts/lib/Strings.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 /**
  * @dev String operations.

contracts/tokens/FxERC1155.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 import {ERC1155} from "../lib/ERC1155.sol";
 import {IFxERC1155} from "./IFxERC1155.sol";
 

contracts/tokens/FxERC20.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import { ERC20 } from "../lib/ERC20.sol";
 import { IFxERC20 } from "./IFxERC20.sol";

contracts/tokens/FxERC721.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 import { ERC721 } from "../lib/ERC721.sol";
 import { IFxERC721 } from "./IFxERC721.sol";

contracts/tokens/IFxERC1155.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 interface IFxERC1155 {
     function fxManager() external returns(address);

contracts/tokens/IFxERC20.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 interface IFxERC20 {
     function fxManager() external returns(address);

contracts/tokens/IFxERC721.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity >=0.7.0<0.8.0;
 
 interface IFxERC721 {
     function fxManager() external returns(address);