diff --git a/.hgignore b/.hgignore
index 194a39e..7998c55 100644
--- a/.hgignore
+++ b/.hgignore
@@ -5,4 +5,4 @@
 .vscode
 docs/.ipynb_checkpoints
 test/*
-thun
+gnu-prolog/thun
diff --git a/thun/gnu-prolog/DCG_basics.pl b/thun/gnu-prolog/DCG_basics.pl
new file mode 100644
index 0000000..b44fd1b
--- /dev/null
+++ b/thun/gnu-prolog/DCG_basics.pl
@@ -0,0 +1,59 @@
+/*
+    Copyright  2019 Simon Forman
+
+    This file is part of Thun
+
+    Thun is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    Thun is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with Thun.  If not see <http://www.gnu.org/licenses/>.
+
+
+DCG basics.  For some of this I cribbed the source from SWI library code
+and adapted it for GNU Prolog Compiler.
+
+N.B. is_glyph//1 excludes '[' and ']' characters.  D'oh!  FIXME
+
+*/
+
+
+:- set_prolog_flag(double_quotes, codes).
+
+
+% TODO: scientific notation.
+
+signed_float_or_integer(Codes) --> signed_digits(J), ".", !, digits(I),
+    { append(J, [0'.|I], Codes) }.
+signed_float_or_integer(Codes) --> signed_digits(Codes).
+
+signed_digits([0'-|Codes]) --> "-", !, digits(Codes).
+signed_digits(     Codes ) -->         digits(Codes).
+
+% Groups of characters.
+
+chars(Chars)   --> one_or_more(char, Chars).
+blanks         --> blank, !, blanks | [].
+digits(Digits) --> one_or_more(digit, Digits).
+
+% Character types.
+
+char(Ch)  --> [Ch], { nonvar(Ch), is_glyph(Ch) }.
+blank     --> [Ch], { nonvar(Ch), is_space(Ch) }.
+digit(Ch) --> [Ch], { nonvar(Ch), between(0'0, 0'9, Ch) }.
+
+is_glyph(Ch) :- Ch =\= 0'[, Ch =\= 0'], between(0'!, 0'~, Ch).
+is_space(Ch) :- Ch =:= 32 ; between(9, 13, Ch).
+
+one_or_more(E, List) --> one_or_more_(List, E).
+
+one_or_more_([Ch|Rest], P) --> call(P, Ch), one_or_more_(Rest, P).
+one_or_more_([Ch],      P) --> call(P, Ch).
+
diff --git a/thun/gnu-prolog/Makefile b/thun/gnu-prolog/Makefile
index 3c702ce..781b1b2 100644
--- a/thun/gnu-prolog/Makefile
+++ b/thun/gnu-prolog/Makefile
@@ -2,14 +2,15 @@
 GPLC_OPTIONS=--no-top-level
 #GPLC_OPTIONS=
 
-thun: thun.pl parser.pl defs.pl main.pl math.pl Makefile
-	gplc $(GPLC_OPTIONS) -o thun thun.pl parser.pl defs.pl main.pl math.pl
+thun: thun.pl parser.pl defs.pl main.pl math.pl DCG_basics.pl Makefile
+	gplc $(GPLC_OPTIONS) -o thun thun.pl parser.pl defs.pl main.pl DCG_basics.pl math.pl
 
-defs.pl: meta-defs.pl parser.pl defs.txt thun.pl
+defs.pl: meta-defs.pl parser.pl defs.txt thun.pl DCG_basics.pl
 	gprolog \
 		--consult-file meta-defs.pl \
 		--consult-file parser.pl \
 		--consult-file thun.pl \
+		--consult-file DCG_basics.pl \
 		--query-goal do
 
 math.pl: meta-math.pl
diff --git a/thun/gnu-prolog/parser.pl b/thun/gnu-prolog/parser.pl
index ecf73da..c60d328 100644
--- a/thun/gnu-prolog/parser.pl
+++ b/thun/gnu-prolog/parser.pl
@@ -37,38 +37,10 @@ symbol(C) --> chars(Chars), !, { Chars \= "==", atom_codes(C, Chars) }.
 num(N) --> number_digits(Codes), { number_codes(N, Codes) }.
 
 number_digits(Codes) --> signed_float_or_integer(Codes), !, end_num.
-% TODO: floats, scientific notation.
-
-signed_float_or_integer(Codes) --> signed_digits(J), ".", !, digits(I),
-    { append(J, [0'.|I], Codes) }.
-signed_float_or_integer(Codes) --> signed_digits(Codes).
-
-signed_digits([45|Codes]) --> "-", !, digits(Codes).
-signed_digits(    Codes ) -->         digits(Codes).
 
 end_num, [Ch] --> [Ch], { [Ch] = "[" ; is_space(Ch) }.
 end_num([], []).
 
-% Groups of characters.
-
-chars(Chars)   --> one_or_more(char, Chars).
-blanks         --> blank, !, blanks | [].
-digits(Digits) --> one_or_more(digit, Digits).
-
-% Character types.
-
-char(Ch)  --> [Ch], { nonvar(Ch), is_glyph(Ch) }.
-blank     --> [Ch], { nonvar(Ch), is_space(Ch) }.
-digit(Ch) --> [Ch], { nonvar(Ch), between(0'0, 0'9, Ch) }.
-
-is_glyph(Ch) :- Ch =\= 0'[, Ch =\= 0'], between(0'!, 0'~, Ch).
-is_space(Ch) :- Ch =:= 32 ; between(9, 13, Ch).
-
-one_or_more(E, List) --> one_or_more_(List, E).
-
-one_or_more_([Ch|Rest], P) --> call(P, Ch), one_or_more_(Rest, P).
-one_or_more_([Ch],      P) --> call(P, Ch).
-
 
 /*