Fix bug in MeetOfPartialPerms

Previously if the argument was empty or of length 1, then this function
would return an error. This commit fixes this, and adds tests.

lib/pperm.gi

@@ -167,20 +167,23 @@ end);
 
 InstallGlobalFunction(MeetOfPartialPerms, 
 function(arg)
-  local meet, i;
-
-  if IsPartialPermCollection(arg[1]) then 
+  local meet, i, empty;
+  
+  if Length(arg) = 1 and IsPartialPermCollection(arg[1]) then 
     return CallFuncList(MeetOfPartialPerms, AsList(arg[1]));
   elif not IsPartialPermCollection(arg) then 
-    Error("usage: the argument should be a collection of partial perms,");
+    Error("usage: the argument should be a collection of partial perms, ");
     return;
   fi;
 
-  meet:=arg[1]; i:=1;
-  repeat
-    i:=i+1;
-    meet:=MEET_PPERMS(meet, arg[i]);
-  until i=Length(arg) or meet=EmptyPartialPerm();
+  meet := arg[1]; 
+  i := 1;
+  empty := EmptyPartialPerm();
+
+  while i < Length(arg) and meet <> empty do
+    i := i + 1;
+    meet := MEET_PPERMS(meet, arg[i]);
+  od;
 
   return meet;
 end);

tst/testinstall/pperm.tst

@@ -670,6 +670,18 @@ gap> g:=PartialPermNC([100001,2,3,4,5]);
 [1,100001](2)(3)(4)(5)
 gap> MeetOfPartialPerms(f,g);
 <identity partial perm on [ 2, 3, 4, 5 ]>
+gap> MeetOfPartialPerms(PartialPerm([1]));
+<identity partial perm on [ 1 ]>
+gap> MeetOfPartialPerms(PartialPerm([1]), PartialPerm([2]));
+<empty partial perm>
+gap> MeetOfPartialPerms(PartialPerm([1]), PartialPerm([1, 2]));
+<identity partial perm on [ 1 ]>
+gap> MeetOfPartialPerms([]);
+Error, usage: the argument should be a collection of partial perms, 
+gap> MeetOfPartialPerms([PartialPerm([1]), PartialPerm([1, 2])]);
+<identity partial perm on [ 1 ]>
+gap> MeetOfPartialPerms(SymmetricInverseMonoid(3));
+<empty partial perm>
 
 # RestrictedPartialPerm
 gap> f:=PartialPermNC([100000,2,3,4,5]);