Performs a calculation on the value of variable a and saves the result back to the variable. If d is true, the type of the variable will be checked and the mapping will be finished with an error if the type is not of a numerical type.

In the case of a division, e specifies the number of digits the result should be rounded to. If nothing or 0 is specified, the result will be rounded to the number of the dividend's digits.

The result of the function is the result of the calculation.