Pattern matching negative vectors sometimes drops the minus sign

[abehring]

I've noticed that the current version 5 of FORM sometimes drops a minus sign when matching a single negative vector. A minimal working example is

S du1;
CF Delta;
V p13,v1,D;

L debug=Delta(-p13,5);

id Delta(v1?,du1?)=D.v1^du1;

Print +s;
.end

With an ancient FORM 3.3 binary I get the expected

       - p13.D^5

while the current master branch yields:

       + p13.D^5

@jodavies also observed that the exact syntax on the right-hand side of the id statement matters for whether the bug is triggered:

• id Delta(v1?) = v1; is OK (removing the second arg)
• id Delta(v1?) = D.v1; too
• id Delta(v1?,du1?) = (D.v1)^du1 too ??

[jodavies]

While discussing this with @abehring I bisected this to c5a24b7c2b476a3c3a65f524d4b14ed4bf3a7059 . It is somewhat surprising that this behaviour has been in the code for so long, without being noticed.

There are several issues on the tracker related to negative vectors, but I don't think any of them are quite the same as this one.