Functions which parse a simple mathematical expression and calculate
the result.

The function mathexp_to_rpn() converts a mathematical expression in 
infix notation, like "3 + 2" to an expression in reverse polish
notation (RPN): "3 2 +". This is easier to calculate, which is done
by the function calculate_rpn(). Operator precedence and parenthesis
is taken into account. There is no checking whether the input is
a valid expression. Dividing is not very precise.

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