5 * 3 + 4  <eval> ...  => 19

(['*'], ['5', '3'])     [on '+']
(['+'], ['15', '4'])    [on eval]

5 + 3 * 4  <eval> ...  => 17

(['+', '*'], ['5', '3', '4'])   [on eval]
(['+'], ['5', '12'])

5 + 3 + 4  <eval> ...  => 12

(['+'], ['5', '3'])     [on second '+']
(['+'], ['8', '4'])     [on eval]

1 + 3 * ( 1 + 3 * 4 )  <eval> ...   => 40

(['+', '*', '(', '+', '*'], ['1', '3', '1', '3', '4'])      [on ')']
(['+', '*', '(', '+'], ['1', '3', '1', '12'])           
(['+', '*'], ['1', '3', '13'])                              [on eval]
(['+'], ['1', '39'])

1 + 3 * ( 1 + 3 * 4  <eval> ...   => *ERROR*


(['+', '*', '(', '+', '*'], ['1', '3', '1', '3', '4'])
(['+', '*', '(', '+'], ['1', '3', '1', '12'])
(['+', '*', '('], ['1', '3', '13'])
(['+', '*'], ['1', '*ERROR*'])
(['+'], ['*ERROR*'])
(['+'], ['*ERROR*', '*ERROR*'])