clarify builder signing domain

[tersec]

No description provided.

[ralexstokes]

@terencechain raised the same issue so if this is not clear then I'm happy to update

the idea is that this follows the semantics of the spec which inherits the python handling of "default" arguments

if we want to make it explicit then I'd suggest specifying the arguments inline like:

compute_domain(DOMAIN_APPLICATION_BUILDER, fork_version=None, genesis_validators_root=None)

what do y'all think?

I generally don't want to stray too far from the pyspec style as I would like to retain the option to write more elaborate specs here in that style where we can, for example, generate conformance tests in a pretty straightforward way

[tersec]

This spec is not, however, written in Python pseudocode. Even after reading that's the intent, it's not something that I'd read into the actual prose.

Nothing about

Signing

All signature operations should follow the [standard BLS operations][bls]
interface defined in consensus-specs.

To assist in signing, we use a function from the [consensus specs][consensus-specs]:
[compute_domain][compute-domain]

There are two types of data to sign over in the Builder API:

• In-protocol messages, e.g. BlindedBeaconBlock, which
  should compute the signing root using [compute_signing_root][compute-root]
  and use the domain specified for beacon block proposals.
• Builder API messages, e.g. validator registration, which should compute the
  signing root using [compute_signing_root][compute-root] with domain given by
  compute_domain(DOMAIN_APPLICATION_BUILDER).
  As compute_signing_root takes SSZObject as input, client software should
  convert in-protocol messages to their SSZ representation to compute the signing
  root and Builder API messages to the SSZ representations defined
  above.

suggests Python semantics on the level of default arguments are more than accidental.

[ralexstokes]

To assist in signing, we use a function from the [consensus specs][consensus-specs]:
[compute_domain][compute-domain]

I think this makes it clear, but like I said I'm open to clarifying further.

Do you like the way I wrote it above or would you rather spell out the data like you did here?

[tersec]

Do you like the way I wrote it above or would you rather spell out the data like you did here?

I do like your suggested approach: 8f68a98

[ralexstokes]

lgtm!