[plt-scheme] reference before it's defined

Sometimes I get the error message
"reference to an identifier before its definition: [name of function]"
and PLT (at intermediate student level) seems to require that the function
is defined before it is used.

But if we want two mutually recursive functions, obviously that won't work.
And also the error message only seems to come up SOMETIMES when we do
this, not consistently.

What am I not understanding about this?

Thanks,
--Joshua Zucker