A label is a user-defined symbol that identifies a location in
    the program. The symbol is assigned a value equal to the location
    counter where the label occurs. The following rules apply when
    coding source statements with labels:

    o  If a statement contains a label, the label must be in the
       first field on the line.

    o  The user-defined symbol name can be up to 31 characters long
       and can contain any alphanumeric character, as well as the
       underscore (_),  dollar sign ($), and period (.)  characters.

    o  If a label extends past column 7, Digital recommends you place
       it on a line by itself so that the following operator field
       can start in column 9 of the next line.

    o  A label is terminated by a colon (:)  or a double colon (::).

    In the following source statement, EXP: is the label field:

    EXP:    .BLKL   50           ; Table stores expected values

    There are three types of labels:

    o  Global labels-Can be referenced by other object modules and
       are defined using a double colon (::).

    o  Local labels-Can be referenced only within the current module
       and are defined using a single colon (:).

    o  Temporary labels-Can be referenced only within the bounds
       of either two local labels, two global labels, two psects
       (program sections), or within the bounds of a temporary
       label scope, as defined by .ENABLE LOCAL_BLOCK and
       .DISABLE LOCAL_BLOCK. Temporary labels are defined using one
       to five digits followed by a dollar sign and a single colon
       ($:).

    The following example shows how these labels appear in use:

    EXP:    .BLKL   50           ; EXP is a local label used to
                                 ; identify a 50-word block of storage
    DATA::  .BLKW   25           ; DATA is a global label used to
                                 ; identify a 25-word block of storage
    10$:    .BLKW    5           ; 10$ is a temporary label used to
                                 ; identify a five word block of
                                 ; storage.