You can look at the Red source code of user-defined and mezzanine functions with the source word.

For example, typing source replace in the REPL shows you the source code of the replace function we encountered in the previous chapter. Here is the output of this command:

replace: func [
       series [series!]
       pattern
       value
       /all
 ...
 ]

It's only about 30 lines, but we have shown only the beginning here; type in the command in the console to see the entire output.

This also works for your own functions—for example, type source fact after you have defined fact in the REPL or in a code file, and you can see its code! You can even use source source.