Value( f, w )
Let f be  a Laurent  polynomial (..., f_{-1}, f_0,  f_1,  ...).  Then
Value returns the finite sum ... +  f_{-1} <w>^{-1} + f_0 <w>^0 +  f_1
<w> + ....
Note that x need not be contained in the base ring of f.
    gap> x := Indeterminate(Integers);; x.name := "x";;
    gap> k := -x + 1;
    -x + 1
    gap> Value( k, 2 );
    -1
    gap> Value( k, [[1,1],[0,1]] );
    [ [ 0, -1 ], [ 0, 0 ] ]
GAP 3.4.4