EXPORT [append, equ].
  IMPORT foo.
  EXPORT grotz.
  grotz :- foo.
  equ( V, V ).
  append( [], L, L ).
  append( [Z|L1], L2, [Z|L3] ) :- append( L1, L2, L3 ).
  printstring( [] ).
  printstring( [H|T] ) :- put( H ), printstring( T ).


  rev( [], [] ).
  rev( [H|T], L ) :- rev( T,Z), append( Z, [H], L ).