For some reason, I can no longer add custom Compose key sequences. I have the file .XCompose in my home folder with content like
<Multi_key> <equal> <asciitilde> : "≅"
<Multi_key> <underscore> <slash> : "∠"
<Multi_key> <slash> <underscore> : "∠"
etc.
Default sequences like the ones that make « á », etc. work.