
  -spec constant_function(Set, AnySet) -> Function
                             when
                                 AnySet :: anyset(),
                                 Function :: a_function(),
                                 Set :: a_set().

  Creates the function that maps each element of set Set onto 
  AnySet.

    1> S = sofs:set([a,b]),
    E = sofs:from_term(1),
    R = sofs:constant_function(S, E),
    sofs:to_external(R).
    [{a,1},{b,1}]
