GTB - convert gray code to binary (execute a gtb).

Usage:

B:
   binary = gtb( gray );

C:
   int gtb(int gray);

Description:

GTB executes a DPS-8 "gtb" instruction. The function interprets its argument as a 36-bit gray code quantity, and returns the conventional twos complement binary equivalent. The primary use of the function is to aid in tracing programs, since the gray-to-binary instruction is so rarely used.