Generic.EqualCustom equal function metadata key.
If a gist or field metadata defines a value for this key, the Fun.Generic.equal function uses it for determining equality.
Using a function that always returns true like Equal.ignore can also be used to indicate that a value or field should not influence the comparison.
val ignore : 'a tignore v0 v1 is true.
val mem : ('a, 'b) Type.Gist.Meta.t2 -> boolval add : 'b t -> ('a, 'b) Type.Gist.Meta.t2 -> ('a, 'b) Type.Gist.Meta.t2val find : ('a, 'b) Type.Gist.Meta.t2 -> 'b t optionval remove : ('a, 'b) Type.Gist.Meta.t2 -> ('a, 'b) Type.Gist.Meta.t2