Invisible XML Grammar

{ Sample grammar adapted from Dick Grune and Ceriel J. H. Jacobs,
    Parsing techniques: A practical guide (New York: Ellis Horwood, 1990;
    second edition New York:  Springer, 2008), pp. 89 (1e) and 112 (2e).

    They use this grammar to illustrate CYK parsing.
    "This grammar describes the syntax of numbers in scientific notation."

    Versions of this grammar after elimination of epsilon-rules, 
    after eliminating unit rules, and after further cleanup (elimination
    of unreachable non-terminals) are given in pp. 120-122 of 2e.  I
    have not made test cases from them.  A Chomsky Normal Form equivalent
    is sample grammar 54.
    }

    { Revisions:
    2018-08-09 : CMSMcQ : made first transcription.
    }

    Number: Integer; Real.
    Integer: Digit; Integer, Digit.
    Real: Integer, Fraction, Scale.
    Fraction:  ".", Integer.
    Scale:  "e", Sign, Integer; Empty.
    Digit:  ["0"-"9"].
    Sign:  "+"; "-".
    Empty: .

    {e.g. 32.5e+1 }

    { Hmm.  Required sign for scale? }