procedure main()
&trace := -1 # ensures functions print their names

every (i := false | true ) & ( j := false | true) do {
  write("i,j := ",image(i),", ",image(j))
  write("i & j:")
  x := i() & j()   # invoke true/false
  write("i | j:")
  y := i() | j()   # invoke true/false
  }
end

procedure true()   #: succeeds always (returning null)
return
end

procedure false()  #: fails always
fail    # for clarity but not needed as running into end has the same effect
end