| how declared | behaviour |
|---|---|
\define, $set, $let, $vars, \procedure, \widget | Every function is a variable, but only variables defined using \function are invokable using the function filter operator. Attempts to use a non-function variable is the same as if the function doesn't exist. The behavior in this case is like the identity function. All filter input is passed unchanged to the output. |
| \function | The body text of the function is treated as a filter expression and evaluated. This filter expression can itself contain a function call. Filter expressions can be factored out into functions arbitrarily deep. |