You should only bill for time spent on client work, unless stated in the contract. When I did consulting, I worked over 40 hours to bill 40 client hours, because there is always some company overhead that isn't related to customer work.
The 40 hours you see billed regularly is maybe to avoid triggering overtime/excess hours clauses or norms.
I must mention Factor, which is a modern concatenative, stack-based programming language. It turns out Forth was just the first concatenative language discovered/constructed.