@code{t}. Otherwise, it returns the longest initial sequence common
to all possible matching completions.
-If @var{collection} is an list, the permissible completions are
+If @var{collection} is a list, the permissible completions are
specified by the elements of the list, each of which should be either
a string, or a cons cell whose @sc{car} is either a string or a symbol
(a symbol is converted to a string using @code{symbol-name}). If the