Added some explanation about how the parser is generated, taken from an email by

Zoltan Boszormenyi <zb@cybertec.at>.

src/interfaces/ecpg/preproc/README.parser

+ECPG modifies and extends the core grammar in a way that
+1) every token in ECPG is <str> type. New tokens are
+   defined in ecpg.tokens, types are defined in ecpg.type
+2) most tokens from the core grammar are simply converted
+   to literals concatenated together to form the SQL string
+   passed to the server, this is done by parse.pl.
+3) some rules need side-effects, actions are either added
+   or completely overridden (compared to the basic token
+   concatenation) for them, these are defined in ecpg.addons,
+   the rules for ecpg.addons are explained below.
+4) new grammar rules are needed for ECPG metacommands.
+   These are in ecpg.trailer.
+5) ecpg.header contains common functions, etc. used by
+   actions for grammar rules.
+
+In "ecpg.addons", every modified rule follows this pattern:
+       ECPG: dumpedtokens postfix
+where "dumpedtokens" is simply tokens from core gram.y's
+rules concatenated together. e.g. if gram.y has this:
+       ruleA: tokenA tokenB tokenC {...}
+then "dumpedtokens" is "ruleAtokenAtokenBtokenC".
+"postfix" above can be:
+a) "block" - the automatic rule created by parse.pl is completely
+    overridden, the code block has to be written completely as
+    it were in a plain bison grammar
+b) "rule" - the automatic rule is extended on, so new syntaxes
+    are accepted for "ruleA". E.g.:
+      ECPG: ruleAtokenAtokenBtokenC rule
+          | tokenD tokenE { action_code; }
+          ...
+    It will be substituted with:
+      ruleA: <original syntax forms and actions up to and including
+                    "tokenA tokenB tokenC">
+             | tokenD tokenE { action_code; }
+             ...
+c) "addon" - the automatic action for the rule (SQL syntax constructed
+    from the tokens concatenated together) is prepended with a new
+    action code part. This code part is written as is's already inside
+    the { ... }
+
+Multiple "addon" or "block" lines may appear together with the
+new code block if the code block is common for those rules.
+