Binary operators should support by clauses

[brian-brazil]

Sometimes the LHS and RHS don't quite have the same labels, but are still matchable 1-to-1. Allowing by clauses on binary operators would allow for this use case.
#393 is a more advanced case where you want to do a 1-to-many match.

[brian-brazil]

More particularly if I have

a{label=foo,otherlabel=z} 1
b{label=foo} 2

then a + by (label) b should produce:

{label=foo} 3

and if I have

a{label=foo,otherlabel=z} 1
a{label=foo,otherlabel=x} 1
b{label=foo} 2

the I'd get an error of some form as this usually indicates an error in the expression (#393 covers the case where you explicitly want this to work).

[beorn7]

Thanks for the explanation.

Since you are planning a similar thing for unary operators AKA functions, how would things work there?

[juliusv]

Done by @fabxc in #554.

[lock]

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