Produces a constant value corresponding to datum (i.e., the representation of the program fragment) without its lexical information, source location, etc. Quoted pairs, vectors, and boxes are immutable.
(#%datum . datum)
Expands to (quote datum), as long as datum is not a keyword. If datum is a keyword, a syntax error is reported.
See also Expansion Steps for information on how the expander introduces #%datum identifiers.