Go to the previous , next section. Keywords are reserved, so don't try to use them as macro names or whatever. You can use the lowercase forms if you explicitly ask to be allowed to overload them. The control words are : WORD STRING FLOAT INTEGER ( ) { 2 \n ! ... Arithmetic words are : ABS ACOS ASIN ATAN ATAN2 CONCAT COS DIMEN EXP FFT INT LG LN PI POWER_SYMBOL SIN SQRT STRLEN SUM TAN WHATIS [ ] = + - * / < > | ... This has nothing to do with their use in subscripting arrays! ...