two types of data: numbers and symbols

THREE KINDS OF NUMBERS

• integer
• float
• Ratios

ORDEROF INPUTS IS IMPORTANT

Symbols are another type of data in Lisp (like Symbols means string)

predicates 断言函数 @logic（&boolean function）

answer T NIL

Everything else is treated as yes

built-in function

Arithmetic functions: +, -, *, /, ABS, SQRT

Predicates: NUMBERP, SYMBOLP, ZEROP, ODDP, EVENP, <, >, EQUAL, NOT