retroforth/doc/book/techniques/stack-diagrams

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# Stack Diagrams
Most words in RETRO have a stack comment. These look like:
(-)
(nn-n)
As with all comments, a stack comment begins with `(` and
should end with a `)`. There are two parts to the comment.
On the left side of the `-` is what the word *consumes*. On
the right is what it *leaves*.
RETRO uses a short notation, with one character per value
taken or left. In general, the following symbols represent
certain types of values.
| Notation | Represents |
| ------------------- | ----------------------- |
| b, n, m, o, x, y, z | generic numeric values |
| s | string |
| v | variable |
| p, a | pointers |
| q | quotation |
| d | dictionary header |
| f | `TRUE` or `FALSE` flag. |
In the case of something like `(xyz-m)`, RETRO expects z to be
on the top of the stack, with y below it and x below the y
value. And after execution, a single value (m) will be left on
the stack.
Words with no stack effect have a comment of (-)