SUB.ST - extract a substring.

Usage:

B:
   sub.st(outstr, instr, length [, start]);

/* C users use "strstr" */

Where:

outstr: points to where the extracted string is to be placed.
instr: points to the string the substring is found in.
length: is the length of the substring to be extracted.
start: is the position of the first character to pick up. The default "start" is zero.

Description:

SUB.ST is used to extract a substring from another string. It picks up "length" characters from "instr" starting at position "start", places a '*0' on the end, and puts the result in "outstr". No checks are done for the actual length of "instr".