If ElemSpec is an atom, it must specify a non-logical variable visible from the caller module.
If ElemSpec is a compound term, it must specify an element visible array. All its arguments must be non negative integer smallers than the bound specified with array/1/2 or variable/1.
Success:
      [eclipse]: local(array(a(4), prolog)),
              setval(a(0), -2),
              decval(a(0)),
              getval(a(0),X).
      X = -3
      yes.
      [eclipse]: local(array(g(4, 5), integer)),
              decval(g(3, 2)),
              getval(g(3, 2), X).
      X = -1
      yes.
      [eclipse]: setval(a, 3),
              decval(a),
              getval(a, X).
      X = 2
      yes.
Error:
      decval(X).                          (Error 4).
      setval(a, 2.0),
          decval(a).                      (Error 5).
      local(array(a(2), float)),
          decval(a(1)).                   (Error 5).
      local(array(a(2))),
          decval(a(2)).                   (Error 6).
      decval(no_var).                     (Error 41).
      decval(noarray(2, 4)).              (Error 41).