
  -spec composite(Function1, Function2) -> Function3
                     when
                         Function1 :: a_function(),
                         Function2 :: a_function(),
                         Function3 :: a_function().

  Returns the composite of the functions Function1 and 
  Function2.

    1> F1 = sofs:a_function([{a,1},{b,2},{c,2}]),
    F2 = sofs:a_function([{1,x},{2,y},{3,z}]),
    F = sofs:composite(F1, F2),
    sofs:to_external(F).
    [{a,x},{b,y},{c,y}]
