What does #+#. mean in lisp?

Question

It is almost impossible to google, hence my understanding is limited to contextual clues from reading through the slime source code: perhaps it is part of the object system in c

Answer 1

They're Common Lisp reader macro characters:

a textual notation introduced by dispatch on one or two characters that defines special-purpose syntax for use by the Lisp reader, and that is implemented by a reader macro function.

Reader macros should not be confused with regular macros - they have nothing to do with each other.

The set-dispatch-macro-character function can be used to extend the Common Lisp syntax with custom reader macros.