page 51, line 15-17:

Frank Yung-Fong Tang pointed out that
the phrase "substituting every symbol x in the list L" on line 15 can
be misleading. It should just say "substituting every symbol in the
list L", or perhaps even more simply "substituting the symbols in the
list L for any occurrence of the symbol s". Also, the following example
(where the initial backquote is missing) with the replace operator -> is
confusing because replacement has not yet been mentioned. It would be
better to use an operator such as crossproduct that has already been
introduced:

`[[a:b], b, x y z] denotes the same relation as [a:x | a:y | a:z].

`[[a:b], b, x y z] denotes the same relation as [a:x | a:y | a:z].