Mathics3 signature pattern matching fails when a function has the FLAT attribute

[rocky]

Description

When a builtin function has the Flat attribute (as happens in the Union builtin), and we have a particular kind of pattern, the matching will not pass a full flattened list.

How to Reproduce

Consider this pattern docstring for Union:

        "Union[lists__, SameTest->sametest_]"

If we encounter:

Union[{1, -1, -2}, {2, 3}, SameTest -> (Abs[#1] == Abs[#2] &)]

and Union has the "Flat" attribute (flatten lists), then the pattern matches, but the eval() method gets passed {1, -1, -2} for list rather than {{1, -1, -2, 2, 3}.

If instead Union does not have the Flat attribute, the pattern matches, and the eval() method gets passed {{1, -1, -2}, {2, 3}} in list.

Context

This behavior is seen in the code. Currently, we omit the flat attribute for Union because of this behavior.

The actual pattern used is a little more complicated since it does not appear in the Union class but instead appears in the Abstract base class _SetOperation as:

"%(name)s[lists__, SameTest->sametest_]"