A schema type  is  a  user-defined  construct  that  provides  a
  template  for  a  family  of distinct data types.  A schema type
  definition contains one or more formal discriminants  that  take
  the   place   of  specific  boundary  values  or  variant-record
  selectors.  By specifying  boundary  or  selector  values  to  a
  schema  type,  you form a valid data type; the provided boundary
  or selector values are called actual discriminants.

  Syntax:

     schema-identifier
        ({discriminant-identifier},... : [[attribute-list]]
          ordinal-type-name};...) = [[attribute-list]] type-denoter;

  The 'schema-identifier' is the name of the schema.

  The  'discriminant-identifier'  is  the   name   of   a   formal
  discriminant.

  The 'ordinal-type-name' is the type of the formal  discriminant,
  which must be an ordinal type.

  The 'attribute-list' is one or  more  identifiers  that  provide
  additional information about the type-denoter.

  The 'type-denoter' is the type definition of the  components  of
  the  schema.   This  must  define  a  new record, array, set, or
  subrange type.

  Each schema type definition requires at least  one  discriminant
  identifier.   A discriminant identifier does not have to be used
  in the type-denoter definition, but it is used to determine type
  compatibility.   Discriminant  identifiers can appear anywhere a
  value is required in this definition.

  TYPE
     Array_Template( Upper_Bound : INTEGER )

  The identifier Upper_Bound is the  formal  discriminant  of  the
  Array_Template  schema.   The  Array_Template  schema  is  not a
  complete description of data.  It is not a valid data type until
  you  provide  an  actual  discriminant that designates the upper
  boundary of the array template.

  Actual   discriminants   can   be   compile-time   or   run-time
  expressions.  This expression must be assignment compatible with
  the ordinal type specified for the formal  discriminant.   Also,
  the actual discriminant value must be inside the range specified
  for the formal discriminant; in the case of subranges, the upper
  value must be greater than or equal to the lower value.