Waterloo Structure BASIC for the Commodore VIC-20 is an extension to the VIC built-in BASIC. Although Waterloo BASIC is usually in cartridge format [VIC-1001 Cartridge], this version is loadable from BASIC into memory location $a000 [decimal 40960], so you'll need a 32k RAM expansion and sys40960 to kick it in.

• Link to the manual