#' is a macro-character which expands #'FOO to (FUNCTION FOO). Symbols in
Lisp have two bindings, one for values and one for functions, allowing them
to represent both variables and functions, depending on context. #'FOO
accesses FOO's lexical function binding in a context where the value
interpretation would normally occur. #' is also used to create lexical
closures for lambda expressions. A lexical closure is a function which when
invoked executes the body of the lambda-expression in the lexical
environment within which the closure was created. See pp. 115-117 of CLtL2
for more details.