Now we examine the accessor readtable-case. This feature reads or sets the upper/lower case behaviour of a readtable. That behaviour affects how symbol names are generated from an input stream.
For instance, the startup readtable-case for the standard readtable in SBCL is :UPCASE. If the REPL is given as input the string Hello, the symbol generated is HELLO.
Other possible behaviours are :DOWNCASE, :PRESERVE, and :INVERT. The first two are self-explanatory. The third one preserves the case for mixed-case strings, but inverts it for single-case strings. So Hello remains Hello, while HELLO becomes hello and hello becomes HELLO. This :INVERT behaviour is not seen in keyword symbols in SBCL 1.1.14: :HELLO remains :HELLO.