A defined operation is unary or binary.  It is defined by a
  function subprogram containing a generic interface block with the
  specifier OPERATOR.  A defined operation is not an intrinsic
  operation.  However, you can use a defined operation to extend the
  meaning of an intrinsic operator.

  For defined unary operations, the function must contain one
  argument.  For defined binary operations, the function must contain
  two arguments.

  Interpretation of the operation is provided by the function that
  defines the operation.

  A Fortran 95/90 defined operator can contain up to 31 letters, and
  is enclosed in periods (.).  Its name cannot be the same name as
  any of the following:

    o The intrinsic operators  .NOT., .AND., .OR., .XOR.,
      .EQV., .NEQV., .EQ., .NE., .GT., .GE., .LT., and .LE.

    o The logical literal constants .TRUE. or .FALSE..

  No two intrinsic operators can follow one another, but an intrinsic
  or binary operator can be followed by a defined unary operator.

  The result of a defined operation can have any type.  The type of
  the result (and its value) must be specified by the defining
  function.

  The following examples show expressions containing defined
  operators:

    .COMPLEMENT. A

    X .PLUS. Y .PLUS. Z

    M * .MINUS. N