MODULE      M4.

IMPORT      Lists.

BASE        Day, Person.
 
CONSTANT    Monday, Tuesday, Wednesday, Thursday, Friday, Saturday,
            Sunday : Day;
            Fred, Bill, Mary : Person.
PREDICATE   Append3 : List(a) * List(a) * List(a) * List(a).
 
Append3(x,y,z,u) <-
            Append(x,y,w) &
            Append(w,z,u).
