Forth stack diagrams (thing)

In Forth, it is customary to explain a word's action by drawing the top of the stack before and after the word's execution. The two halves are separated by `--'. For instance,

   /MOD ( n1 n2 -- mod quot)

means that /MOD takes 2 numbers and returns `mod' (presumably n<sub>1</sub> MOD n<sub>2</sub>, although the stack diagram only implies it if you know some conventions) and `quot' (presumably n₁ DIV n₂).

The space after the open-parentheses is important: it makes the entire stack diagram a legal Forth comment. Stack diagrams are often written in just this format at the definition of the word:

   : ABS ( n -- u)
       dup 0 < if negate then ;