The Return Relative Position of Substring routine searches for
    the first occurrence of a single substring within a source
    string. If STR$POSITION finds the substring, it returns the
    relative position of that substring. If the substring is not
    found, STR$POSITION returns a zero.

    Format

      STR$POSITION  source-string ,substring [,start-position]

  Additional Information (explode) :

• Corresponding JSB Entry Point
• Returns
• Arguments