1. A method of operating a computer system to process an interpretable program, the computer system including an interpreter, and having a processor and a memory, the method comprising the steps of: executing the interpreter on the interpretable program until encountering a calling reference to a separately compiled procedure for accepting one or more arguments and responsively producing some result; reading into the memory a signature file that identifies the arguments and any result of the separately compiled procedure; and generating source code for a call file procedure that accepts from the interpreter the arguments identified in the signature file, passes the arguments to the separately compiled procedure and returns to the interpreter any result identified in the signature file.