Filtering with a Migration

[jpfairbanks]

@KevinDCarlson I was reading the tests and I don't see why this migration filters things. Shouldn't I have to assert that d == l somewhere?

#Filter impossible edges out of a mechanical linkage
M = @migration SchMechLink SchMechLink begin
  V => V
  E => @join begin
          e :: E
          L :: Len
          (l:e→L) :: (x->len(x)^2)
          (d:e→L) :: (x->sum((pos(src(x))-pos(tgt(x))).^2))
      end
  Pos => Pos
  Len => Len
  src => src(e)
  tgt => tgt(e)
  pos => pos
  len => len(e)
end

[KevinDCarlson]

It's a conjunctive migration, so when you run it it takes the limit of that diagram E's mapping to, i.e. precisely the equalizer of d,l!