term (wd,val) (tok:toks) = [(val,toks)], if tok = wd
                         =   [], otherwise
term   x      []         =   []

(p $orelse q) toks = p toks ++ q toks

(p1 $then p2) toks 
      = [((v1,v2),t2) | (v1,t1) <- p1 toks; 
                          (v2,t2) <- p2 t1]

numb = term ("one", 1) $orelse term ("two",2) $orelse term ("three",3)

plus = term ("plus", (+))

add_seq 
    = numb 
      $orelse 
      ((numb $then plus $then add_seq) $apply_rule app_op)

(p $apply_rule f) inp    = [ (f v, r) | (v, r) <- p inp ]

app_op (v1, (op, v2)) = op v1 v2

example = add_seq ["one", "plus", "two", "plus", "three"]