Revision 5939

BACKPORT:  changes from 1.51 and 1.52:

1.52: The copy() method now does not copy values listed by the noinherit()
method (e.g., correct_ans, test_values, etc). This resolves bug #1528.

1.51: Allow the perlFunction method to produce functions that return
formulas when not all the variables are used as arguments.

For example:

    Formula("x+y")->perlFunction('f',['x']);
    $f = f(2);

would produce the equivalent of

    $f = Formula("2+y");

Using the substitute method is more efficient, but this now does the
right thing rather than generate error messages.