diff --git a/polymani.pl b/polymani.pl
index 7afc85c..55cc0d6 100644
--- a/polymani.pl
+++ b/polymani.pl
@@ -187,38 +187,38 @@ nlp_compute(simplify, P) :-
 nlp_compute(_,_) :-
     fail.
 
-special_word_number(zero,       0,       f).
-special_word_number(a,          1,       f).
-special_word_number(one,        1,       f).
-special_word_number(two,        2,       f).
-special_word_number(three,      3,       f).
-special_word_number(four,       4,       f).
-special_word_number(five,       5,       f).
-special_word_number(six,        6,       f).
-special_word_number(seven,      7,       f).
-special_word_number(eight,      8,       f).
-special_word_number(nine,       9,       f).
-special_word_number(ten,        10,      f).
-special_word_number(eleven,     11,      f).
-special_word_number(twelve,     12,      f).
-special_word_number(thirteen,   13,      f).
-special_word_number(fourteen,   14,      f).
-special_word_number(fifteen,    15,      f).
-special_word_number(sixteen,    16,      f).
-special_word_number(seventeen,  17,      f).
-special_word_number(eighteen,   18,      f).
-special_word_number(nineteen,   19,      f).
-special_word_number(twenty,     20,      fy).
-special_word_number(thirty,     30,      fy).
-special_word_number(forty,      40,      fy).
-special_word_number(fifty,      50,      fy).
-special_word_number(sixty,      60,      fy).
-special_word_number(seventy,    70,      fy).
-special_word_number(eighty,     80,      fy).
-special_word_number(ninety,     90,      fy).
-special_word_number(hundred,    100,     xfy).
-special_word_number(thousand,   1000,    xfy).
-special_word_number(million,    1000000, xfy).
+special_word_number(zero,      0,       f).
+special_word_number(a,         1,       f).
+special_word_number(one,       1,       f).
+special_word_number(two,       2,       f).
+special_word_number(three,     3,       f).
+special_word_number(four,      4,       f).
+special_word_number(five,      5,       f).
+special_word_number(six,       6,       f).
+special_word_number(seven,     7,       f).
+special_word_number(eight,     8,       f).
+special_word_number(nine,      9,       f).
+special_word_number(ten,       10,      f).
+special_word_number(eleven,    11,      f).
+special_word_number(twelve,    12,      f).
+special_word_number(thirteen,  13,      f).
+special_word_number(fourteen,  14,      f).
+special_word_number(fifteen,   15,      f).
+special_word_number(sixteen,   16,      f).
+special_word_number(seventeen, 17,      f).
+special_word_number(eighteen,  18,      f).
+special_word_number(nineteen,  19,      f).
+special_word_number(twenty,    20,      fy).
+special_word_number(thirty,    30,      fy).
+special_word_number(forty,     40,      fy).
+special_word_number(fifty,     50,      fy).
+special_word_number(sixty,     60,      fy).
+special_word_number(seventy,   70,      fy).
+special_word_number(eighty,    80,      fy).
+special_word_number(ninety,    90,      fy).
+special_word_number(hundred,   100,     xfy).
+special_word_number(thousand,  1000,    xfy).
+special_word_number(million,   1000000, xfy).
 
 parse_number(void, T, [WN | R], NC) :-
     special_word_number(WN, N, P),
@@ -236,12 +236,15 @@ parse_number(TL, T, [WN | R], NC) :-
     parse_number(op(*, TL, N), T, R, NC),
     !.
 parse_number(TL, T, [and, WN | R], NC) :-
+    special_word_number(WN, _, _),
     parse_number(TL, T, [WN | R], NC),
     !.
 parse_number(T, T, [WN | R], [WN | R]) :-
-    not(word_tree(WN, _)),
+    T \= void,
+    not(special_word_number(WN, _, _)),
     !.
 parse_number(T, T, [], []) :-
+    T \= void,
     !.
 %% Tests:
 %% ?- parse_number(void, T, [two], _).
@@ -262,14 +265,47 @@ parse_number(T, T, [], []) :-
 %@ T = op(+, op(*, 2, 100), 1).
 %% ?- parse_number(void, T, [twenty, one, hundred, and, twenty, one], _).
 %@ T = op(+, op(+, op(*, op(+, 20, 1), 100), 20), 1).
-%% ?- parse_number(void, T, [twenty, one, hundred, and, twenty, one, foo, bar, blah], _).
-%@ T = op(+, op(+, op(*, op(+, 20, 1), 100), 20), 1).
-%% ?- parse_number(void, T, [twenty, one, hundred, and, bleg, twenty, quux, one, foo, bar], _).
-%@ T = op(*, op(+, 20, 1), 100).
+%% ?- parse_number(void, T, [twenty, one, hundred, and, twenty, one, foo, bar, blah], NC).
+%@ T = op(+, op(+, op(*, op(+, 20, 1), 100), 20), 1),
+%@ NC = [foo, bar, blah].
+%% ?- parse_number(void, T, [twenty, one, hundred, and, bleg, twenty, quux, one, foo, bar], NC).
+%@ T = op(*, op(+, 20, 1), 100),
+%@ NC = [and, bleg, twenty, quux, one, foo, bar].
 %% ?- parse_number(void, T, [two, hundred, thousand], _).
 %@ T = op(*, op(*, 2, 100), 1000).
+%% ?- parse_number(void, T, [twenty, one, hundred, thousand], _).
+%@ T = op(*, op(*, op(+, 20, 1), 100), 1000).
+%% ?- parse_number(void, T, [thirty, five, million], _).
+%@ T = op(*, op(+, 30, 5), 1000000).
+%% ?- parse_number(void, T, [foo, five, million], NC).
+%@ false.
 
+operations(times, *).
+operations(plus,  +).
 
+parse_operation(Op) --> [WOp], { operations(WOp, Op) }.
+
+parse_expression(TLP-TL, op(Op, TLP-TL, TR)) --> parse_number(void, TL),
+						 parse_operation(Op),
+						 parse_expression(op(Op, TL, TRP)-TRP, TR).
+parse_expression(void, T) --> parse_number(void, T), { T \= void }.
+parse_expression(TL, TL-TR) --> parse_number(void, TR), { TR \= void, TL \= void }.
+%% Tests:
+%% ?- parse_expression(T, [], _).
+%@ false.
+%% ?- parse_expression(T, [two], _).
+%@ T = 2.
+%% ?- parse_expression(void, T, [two, times, two], _).
+%@ T = 2 ;
+%@ false.
+%@ T = 2 ;
+%@ false.
+%@ T = op(*, 2, 2).
+%% ?- parse_expression(T, [two, times, three, plus, two], _).
+%@ T = op(*, 2, 3).
+%@ T = op(*, 2, op(+, 3, 2)).
+%% ?- parse_expression(T, [two, times, times, two], _).
+%@ false.