INDEX (string, substring [,back] [,kind])

  Class:  Elemental function - Generic

  Returns the starting position of the substring as an INTEGER*4 or
  INTEGER*8 value.

  "string" and "substring" are of type character, "back" is of type
  logical, and "kind" is a scalar integer initialization expression.

  The result is of type integer.  If "kind" is present, the kind
  parameter of the result is that specified by "kind"; otherwise, the
  kind parameter of the result is that of default integer.  If the
  processor cannot represent the result value in the kind of the
  result, the result is undefined.

  If "back" is absent or false, the leftmost substring is found. If
  "back" is true, the rightmost substring is found.

  Examples:

  INDEX ('FORTRAN', 'O', BACK = .TRUE.) has the value 2.

  INDEX ('XXXX', " ", BACK = .TRUE.) has the value 0.

  INDEX ('XXXX', "", BACK = .TRUE.) has the value 5.