SUBSTR(string,position[,length[,pad]])

This function makes a substring of string. Usually the parameters
string, position and length are supplied and the result is the
substring of string of length "length" starting at the "position"th
character - so
   substr("abcdefg",3,2) = "cd".
If the length is omitted, then the substring continues to the end of the
string - so
   substr("abcdefg",4) = "defg".
If part of the substring requested is outside of string, then spaces are
added - or if the parameter pad is given, this character is used to pad
the result. Thus
   substr("abc",2,4,"0") = "bc00" and substr("abc",-1) = "  abc".

LOCAL: REXX/imc allows the position to be a negative whole number.