Lexical string symbols are similar in concept and syntax to
    numeric symbols. MACRO-64 supports numeric symbols using the
    following syntax:

    numeric_symbol_name = numeric_expression

    MACRO-64 supports lexical string symbols using the following
    syntax:

    string_symbol_name = "any string of characters"

    The assembler differentiates between numeric symbol assignment
    and lexical string symbol assignment as follows:

    o  In a lexical string symbol assignment, a quoted string literal
       must appear after the equal sign.

    o  A lexical string symbol value is specified by the quoted
       string literal.

    The quotes are not included in the value of the lexical string
    symbol. You cannot use the same name for a lexical string symbol,
    numeric symbol, or label.

    Like numeric symbols, lexical string symbols are assembly time
    variables. After you assign a string value to a lexical string
    symbol, you can reassign a different value to that symbol later
    in the assembly.

    You can use lexical string symbols as arguments to lexical
    operators. In particular, you can use a lexical string symbol
    as an argument to the lexical substitution operator (%)  or the
    %STRING lexical operator to substitute the value of the lexical
    string symbol at any point in the text of your program.