Performs subtraction of $a and $b. A can be an array and $b a number, in which case each member of $a gets subtracted $b. If $a is an array and $b is not provided then neg. array_sum is used to return a single numeric value. If both $a and $b are arrays, each member of $a is summed against the corresponding member in $b compared using index.
First number for calculation
If TRUE, throws an Exception if argument "a" is not specified and no child content or inline argument is found. Usually okay to use a NULL value (as integer zero).
Optional: Second number or Iterator/Traversable/Array for calculation