*-1.4.3 数学表达式

  * 三类：关系运算；逻辑运算；算术运算
  
    help operator  //运算符
	
  * 关系运算符  ==;  >;  <;  >=;  <=;  !=;  ~=
    sysuse "auto", clear
    list price if foreign == 0
    sum  price if foreign != 1

  * 逻辑运算符： & -->(与) ;  | -->(或)
    sysuse "auto", clear
    sum price wei if (foreign==1 & rep78<=3)
    sum price wei if (rep78==1) | (rep78==5) | (foreign !=0)
    sum price wei if (rep78>2 & rep78<5) | (price>10000)

  * 算术运算符：+ - * / ^(幂)
    display 5^2
    dis     1 - 3*2 + 4/5 - 9^3 
    dis     2*_pi