My .XCompose file

.XCompose

include "/usr/share/X11/locale/en_US.UTF-8/Compose"

## ^^ go look in that file for compose sequences, this only contains my additions

## Note that this isn't very useful unless you have actually mapped a keyboard key to Multi_key. 
## In my case I use a KDE setting for that

<Multi_key> <less> <equal>      : "⇐" U21D0 # LEFTWARDS DOUBLE ARROW
<Multi_key> <equal> <greater>   : "⇒" U21D2 # RIGHTWARDS DOUBLE ARROW
<Multi_key> <apostrophe> <p>    : "′" U2032 # Prime
<Multi_key> <apostrophe> <P>    : "″" U2033 # Double prime
<Multi_key> <a> <r> <r>         : "☠" U2620 # SKULL AND CROSSBONES
<Multi_key> <bar> <bar>         : "⋮" U22EE # Vertical ellipsis
<Multi_key> <s> <n> <o> <w>     : "☃" U2603 # Snowman
<Multi_key> <t> <f>             : "(╯°□°)╯︵ ┻━┻"
<Multi_key> <p> <o> <o>         : "💩"
<Multi_key> <asciitilde> <asciitilde>     : "≈"
<Multi_key> <asciitilde> <equal>          : "≃"