+# The end of the directives to parse.
+my $eod = "end of directives";
+# The list of tokens parsed by the following functions.
+my @token;
+
+# eat ($EXPECTED)
+# ---------------
+# Check that the current token is $EXPECTED, and move to the next.
+sub eat ($)
+{
+ my ($expected) = @_;
+ die "expected $expected, unexpected: $token[0] (@token)\n"
+ unless $token[0] eq $expected;
+ shift @token;
+}
+
+# Parse directive specifications:
+# expr: term (| term)*
+# term: fact (& fact)*
+# fact: ( expr ) | [ expr ] | dirs
+# dirs: %s SKELETON | #d NAME[=VALUE] | %d NAME[=VALUE] | directive
+sub parse (@)
+{
+ @token = (@_, $eod);
+ verbose 3, "Parsing: @token\n";
+ my @res = parse_expr ();
+ eat ($eod);
+ return @res;
+}
+
+sub parse_expr ()
+{
+ my @res = parse_term ();
+ while ($token[0] eq '|')
+ {
+ eat ('|');
+ # Alternation.
+ push @res, parse_term ();
+ }
+ return @res;
+}
+
+sub parse_term ()
+{
+ my @res = parse_fact ();
+ while ($token[0] eq '&')
+ {
+ eat ('&');
+ # Cartesian product.
+ my @lhs = @res;
+ @res = ();
+ for my $rhs (parse_fact ())
+ {
+ for my $lhs (@lhs)
+ {
+ push @res, $lhs . ($lhs && $rhs ? "\n" : "") . $rhs;
+ }
+ }
+ }
+ return @res;
+}
+
+sub parse_fact ()
+{
+ my @res;
+ die "unexpected end of expression"
+ unless defined $token[0];
+
+ if ($token[0] eq '(')
+ {
+ eat ('(');
+ @res = parse_expr ();
+ eat (')');
+ }
+ elsif ($token[0] eq '[')
+ {
+ eat ('[');
+ @res = (parse_expr (), '');
+ eat (']');
+ }
+ else
+ {
+ @res = parse_dirs ();
+ }
+ return @res;
+}
+
+sub parse_dirs ()
+{
+ my @res;
+ die "unexpected end of expression"
+ unless defined $token[0];
+
+ if ($token[0] eq '#d')
+ {
+ eat ('#d');
+ $token[0] =~ s/(.*?)=(.*)/$1 $2/;
+ @res = ("%code {\n#define $token[0]\n}");
+ shift @token;
+ }
+ elsif ($token[0] eq '%d')
+ {
+ shift @token;
+ $token[0] =~ s/(.*?)=(.*)/$1 "$2"/;
+ @res = ("%define $token[0]");
+ shift @token;
+ }
+ elsif ($token[0] eq '%s')
+ {
+ shift @token;
+ @res = ("%skeleton \"$token[0]\"");
+ shift @token;
+ }
+ else
+ {
+ @res = $token[0];
+ shift @token;
+ }
+
+ return @res;
+}
+
+######################################################################
+