MID$ on the right:

      str-var =  MID[$] (str-exp, int-exp1, int-exp2)

      MID$ on the left:

      MID[$] (str-var, int-exp [,int-exp]) = str-exp

  Int-exp1 specifies  the  starting  position  of  the  substring's  first
  character and int-exp2 specifies the substring's length.  If int-exp1 is
  less than one, BASIC assumes a starting character position of  one.   If
  int-exp2 is less than or equal to zero, BASIC assumes a length of zero.