subtract is:exact

the same as flip (-). Because - is treated specially in the Haskell grammar, (- e) is not a section, but an application of prefix negation. However, (subtract exp) is equivalent to the disallowed section.
Decrease the gauge by the given amount.
subtract is (-) with swapped operand order. This is the operand order which will be needed in most cases of partial application.
subtracts the contents of the bins of the second histogram from the first
subtracts the contents of the bins of the second histogram from the first