Since IB6 or before (can't remember) isql has extracted first the procedure's headers (parameters) and later the bodies. It typically does:
CREATE PROCEDURE P AS
BEGIN EXIT; END ^

However, as FB2.1 has a way to distinguish between selectable and executable procedures and enforces the difference, if a proc A that calls a selectable proc B is defined before B, A won't compile because (due to the header) FB knows it's not selectable.
The solution is that for FB2.1 and newer, isql read the rdb$procedure_type field and do the correct header extraction as shown here:
CREATE PROCEDURE P AS
BEGIN SUSPEND; END ^