"2 3 4 * 5 / +"  --> 2 + 3 * 4 / 5
"3 4 * 5 / 2 +"  --> 3 * 4 / 5 + 2
"3 4 5 / * 2 +"  --> 3 * 4 / 5 + 2
"2 3 + 4 * 5 /"  --> (2 + 3) * 4 / 5
"2 3 4 - -"      --> 2 - (3 - 4)
"2 3 - 4 -"      --> (2 - 3) - 4
"2 3 4 + -"      --> 2 - (3 + 4)
"2 3 4 - +"      --> 2 + 3 - 4