EXPRESSIONS

  An expression is a mixture of symbols which resolves to a value when
  evaluated. In other words, an expression is basically a formula. An
  expression can consist of constants, variables, function calls, and
  operators. An expression can be very simple, or very complicated.
  For example, some expressions are:

       10 + 3 - 5
       9 * 7 / 63 - 30
       result = 10 * max(a, b)
       month >= 10
       200
       command == "bye"
       prints("Hello")

  In an expression, the data being acted upon are constants, vari-
  ables, and functions calls, while the operators (+, *, etc.) are the
  symbols that do things with the data. There are many different oper-
  ators, of which there are two basic types. Binary operators (such as
  +, *, /) perform a calculation on the expression on either side of
  them. Unary operators appear before a single expression and work on
  that.