Read one Lisp expression as text from STREAM, return as Lisp object.
If STREAM is nil, use the value of `standard-input' (which see).
STREAM or the value of `standard-input' may be:
a buffer (read from point and advance it)
a marker (read from where it points and advance it)
a function (call it with no arguments for each character,
call it with a char as argument to push a char back)
a string (takes text from string, starting at the beginning)
t (read text line using minibuffer and use it, or read from
standard input in batch mode). (fn &optional STREAM)