Reading the LRM , Language Reference Manual, syntax.
   ( LRM is the old salts name for ANSI/MIL-STD-1815A )

This will use as an example, subtype.
 1. Use the index in appendix E, on page E-13 subtype is shown
    to have only one entry, subtype_declaration in 3.3.2
    Note: The 3.3.2 is syntax summary location and the LRM 
          paragraph number for reading about subtypes.

 2. Now find 3.3.2 in the middle of the right hand column of
    page E-1. The syntax is repeated here:

    subtype_declaration ::=
      subtype identifier is subtype_indication ;

    subtype_indication ::= type_mark [ constraint ]

    type_mark ::= type_name | subtype_name

    constraint ::=  range_constraint         | floating_point_constraint
                 |  fixed_point_constraint   | index_constraint
                 |  discriminant_constraint  

 3. Just an aside on cross referencing. Look up type_mark on page E-14
    and note that it points back to subtype_indication in 3.3.2

 4. Note that type_name does not appear in the index thus assume it
    is a terminal symbol and use an English definition, e.g. the name
    of a type. Similarily for subtype_name.

 5. The next token to be expanded is range_constraint.
    Looking up range_constraint on page E-12  shows 3.5 as the defining
    paragraph. Note also we can pick up some other references at the same
    time. i.e. fixed_point_constraint 3.5.9, and floating_point_constraint
    3.5.7
    Continuing to collect references, index_constraint 3.6,
    discriminant_constraint 3.7.2
  
 6. Now filling in the expansion from the references we have the following

    range_constraint ::=
    floating_point_constraint ::=
    fixed_point_constraint ::=
    index_constraint ::=
    discriminant_constraint ::=