5.7 Variables
  A variable is a storage location that is referred to by a vari-
  able name. Variable values can change during program
  execution. Each named location can hold only one value at a
  time.

  A variable name can have up to 31 characters. The name
  must begin with a letter; the remaining characters, if any,
  can be any combination of letters, digits, dollar signs ( $ ),
  underscores ( _ ), and periods ( . ).

  Variables can be grouped in an orderly series under a sin-
  gle name, called arrays . You refer to a single variable in
  an array by using one or more subscripts that specify the
  variable's position in the array.