You can use the lexical substitution operator at any point in
    your program to cause the assembler to substitute the value of
    a lexical string symbol for the name of the symbol. The lexical
    substitution operator is the percent sign (%). Place the lexical
    substitution operator to the left and right of the name of the
    lexical string symbol that you wish to subsitute, as follows:

    %lexsym_name%

    For example:

         HORSES    = "All the king's horses"
         MEN       = "all the king's men"
         .print "%HORSES% and %MEN%"

    This example defines two lexical string symbols: HORSES and MEN.
    The third statement displays a message at assembly time. The text
    of the message specifies that the value of the HORSES and MEN
    lexical string symbols be substituted as indicated. After lexical
    processing, the third statement appears as:

         .print "All the king's horses and all the king's men"