There could be a lot more compose sequences than there are. 
Here's a patch that adds some vulgar fractions, superscript numbers,
characters used in Esperanto, and a few random other characters that seemed
like a good idea at the time. 

It occurs to me that we should probably follow some sort of standard for
these. It also occurs to me that it is likely that no such standard exists,
and we should fall back to well thought out design principles for what
compose sequences should be included. It also occurs to me that these also
may not as yet exist.

--- gtk+-2.0.5/gtk/gtkimcontextsimple.c	Wed Jun 12 11:16:36 2002
+++ gtk+-2.0.5.t/gtk/gtkimcontextsimple.c	Fri Jul 19 10:24:58 2002
@@ -235,9 +235,11 @@
   GDK_Multi_key,	GDK_parenleft,	GDK_parenleft,	0,
0,
0x005B,
/* LEFT_SQUARE_BRACKET */
   GDK_Multi_key,	GDK_parenleft,	GDK_minus,	0,
0,
0x007B,
/* LEFT_CURLY_BRACKET */
   GDK_Multi_key,	GDK_parenleft,	GDK_G,	0,
0,
0x011E,
/* LATIN_CAPITAL_LETTER_G_WITH_BREVE */
+  GDK_Multi_key,	GDK_parenleft,	GDK_U,	0,
0,
0x016C,
/* LATIN_CAPITAL_LETTER_U_WITH_BREVE */
   GDK_Multi_key,	GDK_parenleft,	GDK_c,	0,
0,
0x00A9,
/* COPYRIGHT_SIGN */
   GDK_Multi_key,	GDK_parenleft,	GDK_g,	0,
0,
0x011F,
/* LATIN_SMALL_LETTER_G_WITH_BREVE */
   GDK_Multi_key,	GDK_parenleft,	GDK_r,	0,
0,
0x00AE,
/* REGISTERED_SIGN */
+  GDK_Multi_key,	GDK_parenleft,	GDK_u,	0,
0,
0x016C,
/* LATIN_SMALL_LETTER_U_WITH_BREVE */
   GDK_Multi_key,	GDK_parenright,	GDK_parenright,	0,
0,
0x005D,
/* RIGHT_SQUARE_BRACKET */
   GDK_Multi_key,	GDK_parenright,	GDK_minus,	0,
0,
0x007D,
/* RIGHT_CURLY_BRACKET */
   GDK_Multi_key,	GDK_asterisk,	GDK_0,	0,
0,
0x00B0,
/* DEGREE_SIGN */
@@ -338,24 +340,64 @@
   GDK_Multi_key,	GDK_0,	GDK_c,	0,
0,
0x00A9,
/* COPYRIGHT_SIGN */
   GDK_Multi_key,	GDK_0,	GDK_s,	0,
0,
0x00A7,
/* SECTION_SIGN */
   GDK_Multi_key,	GDK_0,	GDK_x,	0,
0,
0x00A4,
/* CURRENCY_SIGN */
+  GDK_Multi_key,	GDK_1,	GDK_slash,	0,
0,
0x215F,
/* FRACTION NUMERATOR ONE */
+  GDK_Multi_key,	GDK_1,	GDK_2,	0,
0,
0x00BD,
/* VULGAR_FRACTION_ONE_HALF */
+  GDK_Multi_key,	GDK_1,	GDK_3,	0,
0,
0x2153,
/* VULGAR_FRACTION_ONE_THIRD */
+  GDK_Multi_key,	GDK_1,	GDK_4,	0,
0,
0x00BC,
/* VULGAR_FRACTION_ONE_QUARTER */
+  GDK_Multi_key,	GDK_1,	GDK_5,	0,
0,
0x2155,
/* VULGAR_FRACTION_ONE_FIFTH */
+  GDK_Multi_key,	GDK_1,	GDK_6,	0,
0,
0x2159,
/* VULGAR_FRACTION_ONE_SIXTH */
+  GDK_Multi_key,	GDK_1,	GDK_8,	0,
0,
0x215B,
/* VULGAR_FRACTION_ONE_EIGHTH */
   GDK_Multi_key,	GDK_1,	GDK_S,	0,
0,
0x00B9,
/* SUPERSCRIPT_ONE */
   GDK_Multi_key,	GDK_1,	GDK_asciicircum,	0,
0,
0x00B9,
/* SUPERSCRIPT_ONE */
   GDK_Multi_key,	GDK_1,	GDK_s,	0,
0,
0x00B9,
/* SUPERSCRIPT_ONE */
+  GDK_Multi_key,	GDK_2,	GDK_3,	0,
0,
0x2154,
/* VULGAR FRACTION TWO THIRDS */
+  GDK_Multi_key,	GDK_2,	GDK_5,	0,
0,
0x2156,
/* VULGAR FRACTION TWO FIFTHS */
   GDK_Multi_key,	GDK_2,	GDK_S,	0,
0,
0x00B2,
/* SUPERSCRIPT_TWO */
   GDK_Multi_key,	GDK_2,	GDK_asciicircum,	0,
0,
0x00B2,
/* SUPERSCRIPT_TWO */
   GDK_Multi_key,	GDK_2,	GDK_s,	0,
0,
0x00B2,
/* SUPERSCRIPT_TWO */
+  GDK_Multi_key,	GDK_3,	GDK_4,	0,
0,
0x00BE,
/* VULGAR FRACTION THREE QUARTERS */
+  GDK_Multi_key,	GDK_3,	GDK_5,	0,
0,
0x2157,
/* VULGAR FRACTION THREE FIFTHS */
+  GDK_Multi_key,	GDK_3,	GDK_8,	0,
0,
0x215C,
/* VULGAR FRACTION THREE EIGHTHS */
   GDK_Multi_key,	GDK_3,	GDK_S,	0,
0,
0x00B3,
/* SUPERSCRIPT_THREE */
   GDK_Multi_key,	GDK_3,	GDK_asciicircum,	0,
0,
0x00B3,
/* SUPERSCRIPT_THREE */
   GDK_Multi_key,	GDK_3,	GDK_s,	0,
0,
0x00B3,
/* SUPERSCRIPT_THREE */
+  GDK_Multi_key,	GDK_4,	GDK_5,	0,
0,
0x2158,
/* VULGAR FRACTION FOUR FIFTHS */
+  GDK_Multi_key,	GDK_4,	GDK_S,	0,
0,
0x2074,
/* SUPERSCRIPT_FOUR */
+  GDK_Multi_key,	GDK_4,	GDK_asciicircum,	0,
0,
0x2074,
/* SUPERSCRIPT_FOUR */
+  GDK_Multi_key,	GDK_4,	GDK_s,	0,
0,
0x2074,
/* SUPERSCRIPT_FOUR */
+  GDK_Multi_key,	GDK_5,	GDK_6,	0,
0,
0x215A,
/* VULGAR FRACTION FIVE SIXTHS */
+  GDK_Multi_key,	GDK_5,	GDK_8,	0,
0,
0x215D,
/* VULGAR FRACTION FIVE EIGHTHS */
+  GDK_Multi_key,	GDK_5,	GDK_S,	0,
0,
0x2075,
/* SUPERSCRIPT_FIVE */
+  GDK_Multi_key,	GDK_5,	GDK_asciicircum,	0,
0,
0x2075,
/* SUPERSCRIPT_FIVE */
+  GDK_Multi_key,	GDK_5,	GDK_s,	0,
0,
0x2076,
/* SUPERSCRIPT_FIVE */
+  GDK_Multi_key,	GDK_6,	GDK_S,	0,
0,
0x2076,
/* SUPERSCRIPT_SIX */
+  GDK_Multi_key,	GDK_6,	GDK_asciicircum,	0,
0,
0x2076,
/* SUPERSCRIPT_SIX */
+  GDK_Multi_key,	GDK_6,	GDK_s,	0,
0,
0x2076,
/* SUPERSCRIPT_SIX */
+  GDK_Multi_key,	GDK_7,	GDK_8,	0,
0,
0x215E,
/* VULGAR FRACTION SEVEN EIGHTHS */
+  GDK_Multi_key,	GDK_7,	GDK_S,	0,
0,
0x2077,
/* SUPERSCRIPT_SEVEN */
+  GDK_Multi_key,	GDK_7,	GDK_asciicircum,	0,
0,
0x2077,
/* SUPERSCRIPT_SEVEN */
+  GDK_Multi_key,	GDK_7,	GDK_s,	0,
0,
0x2077,
/* SUPERSCRIPT_SEVEN */
+  GDK_Multi_key,	GDK_8,	GDK_8,	0,
0,
0x221E,
/* INFINITY */
+  GDK_Multi_key,	GDK_8,	GDK_S,	0,
0,
0x2078,
/* SUPERSCRIPT_EIGHT */
+  GDK_Multi_key,	GDK_8,	GDK_asciicircum,	0,
0,
0x2078,
/* SUPERSCRIPT_EIGHT */
+  GDK_Multi_key,	GDK_8,	GDK_s,	0,
0,
0x2078,
/* SUPERSCRIPT_EIGHT */
+  GDK_Multi_key,	GDK_9,	GDK_S,	0,
0,
0x2079,
/* SUPERSCRIPT_NINE */
+  GDK_Multi_key,	GDK_9,	GDK_asciicircum,	0,
0,
0x2079,
/* SUPERSCRIPT_NINE */
+  GDK_Multi_key,	GDK_9,	GDK_s,	0,
0,
0x2079,
/* SUPERSCRIPT_NINE */
+  GDK_Multi_key,	GDK_colon,	GDK_parenleft,	0,
0,
0x2639,
/* WHITE_FROWNING_FACE */
+  GDK_Multi_key,	GDK_colon,	GDK_parenright,	0,
0,
0x263A,
/* WHITE_SMILING_FACE */
   GDK_Multi_key,	GDK_colon,	GDK_minus,	0,
0,
0x00F7,
/* DIVISION_SIGN */
   GDK_Multi_key,	GDK_less,	GDK_slash,	0,
0,
0x005C,
/* REVERSE_SOLIDUS */
   GDK_Multi_key,	GDK_less,	GDK_less,	0,
0,
0x00AB,
/* LEFTxPOINTING_DOUBLE_ANGLE_QUOTATION_MARK */
+  GDK_Multi_key,	GDK_less,	GDK_equal,	0,
0,
0x2264,
/* LESS-THAN_OR_EQUAL_TO */
   GDK_Multi_key,	GDK_less,	GDK_C,	0,
0,
0x010C,
/* LATIN_CAPITAL_LETTER_C_WITH_CARON */
   GDK_Multi_key,	GDK_less,	GDK_S,	0,
0,
0x0160,
/* LATIN_CAPITAL_LETTER_S_WITH_CARON */
   GDK_Multi_key,	GDK_less,	GDK_Z,	0,
0,
0x017D,
/* LATIN_CAPITAL_LETTER_Z_WITH_CARON */
   GDK_Multi_key,	GDK_less,	GDK_c,	0,
0,
0x010D,
/* LATIN_SMALL_LETTER_C_WITH_CARON */
   GDK_Multi_key,	GDK_less,	GDK_s,	0,
0,
0x0161,
/* LATIN_SMALL_LETTER_S_WITH_CARON */
   GDK_Multi_key,	GDK_less,	GDK_z,	0,
0,
0x017E,
/* LATIN_SMALL_LETTER_Z_WITH_CARON */
+  GDK_Multi_key,	GDK_equal,	GDK_less,	0,
0,
0x2264,
/* LESS-THAN_OR_EQUAL_TO */
+  GDK_Multi_key,	GDK_equal,	GDK_greater,	0,
0,
0x2265,
/* GREATER-THAN_OR_EQUAL_TO */
   GDK_Multi_key,	GDK_equal,	GDK_C,	0,
0,
0x20AC,
/* EURO_SIGN */
   GDK_Multi_key,	GDK_equal,	GDK_L,	0,
0,
0x00A3,
/* POUND_SIGN */
   GDK_Multi_key,	GDK_equal,	GDK_Y,	0,
0,
0x00A5,
/* YEN_SIGN */
@@ -363,6 +405,7 @@
   GDK_Multi_key,	GDK_equal,	GDK_l,	0,
0,
0x00A3,
/* POUND_SIGN */
   GDK_Multi_key,	GDK_equal,	GDK_y,	0,
0,
0x00A5,
/* YEN_SIGN */
   GDK_Multi_key,	GDK_greater,	GDK_space,	0,
0,
0x005E,
/* CIRCUMFLEX_ACCENT */
+  GDK_Multi_key,	GDK_greater,	GDK_equal,	0,
0,
0x2265,
/* GREATER-THAN_OR_EQUAL_TO */
   GDK_Multi_key,	GDK_greater,	GDK_greater,	0,
0,
0x00BB,
/* RIGHTxPOINTING_DOUBLE_ANGLE_QUOTATION_MARK */
   GDK_Multi_key,	GDK_greater,	GDK_A,	0,
0,
0x00C2,
/* LATIN_CAPITAL_LETTER_A_WITH_CIRCUMFLEX */
   GDK_Multi_key,	GDK_greater,	GDK_E,	0,
0,
0x00CA,
/* LATIN_CAPITAL_LETTER_E_WITH_CIRCUMFLEX */
@@ -397,6 +440,7 @@
   GDK_Multi_key,	GDK_C,	GDK_less,	0,
0,
0x010C,
/* LATIN_CAPITAL_LETTER_C_WITH_CARON */
   GDK_Multi_key,	GDK_C,	GDK_equal,	0,
0,
0x20AC,
/* EURO_SIGN */
   GDK_Multi_key,	GDK_C,	GDK_O,	0,
0,
0x00A9,
/* COPYRIGHT_SIGN */
+  GDK_Multi_key,	GDK_C,	GDK_asciicircum,	0,
0,
0x0108,
/* LATIN_CAPITAL_LETTER_C_WITH_CIRCUMFLEX */
   GDK_Multi_key,	GDK_C,	GDK_o,	0,
0,
0x00A9,
/* COPYRIGHT_SIGN */
   GDK_Multi_key,	GDK_C,	GDK_bar,	0,
0,
0x00A2,
/* CENT_SIGN */
   GDK_Multi_key,	GDK_D,	GDK_minus,	0,
0,
0x0110,
/* LATIN_CAPITAL_LETTER_D_WITH_STROKE */
@@ -417,7 +461,9 @@
   GDK_Multi_key,	GDK_G,	GDK_comma,	0,
0,
0x0122,
/* LATIN_CAPITAL_LETTER_G_WITH_CEDILLA */
   GDK_Multi_key,	GDK_G,	GDK_period,	0,
0,
0x012