A special-sequence consists of a special-sequence-symbol
followed by a (possibly empty) sequence of special-sequence-
characters followed by a special-sequence-symbol.
A syntactic-primary consists of one of the following:
(a) an optional-sequence;
(b) a repeated-sequence;
(c) a grouped-sequence;
(d) a meta-identifier;
(e) a terminal-string;
(f) a special-sequence;
(g) an empty-sequence.
A syntax-rule consists of a meta-identifier (the name of
the non-terminal symbol being defined) followed by a
defining-symbol followed by a definitions-list followed by
a terminator-symbol.