If a specification fails to match at some point, this does not necessarily mean a syntax error will be signaled; instead, backtracking will take place until all alternatives have been exhausted. Eventually every element of the argument list must be matched by some element in the specification, and every required element in the specification must match some argument.
When a syntax error is detected, it might not be reported until much
later, after higher-level alternatives have been exhausted, and with the
point positioned further from the real error. But if backtracking is
disabled when an error occurs, it can be reported immediately. Note
that backtracking is also reenabled automatically in several situations;
when a new alternative is established by &optional,
&rest, or &or, or at the start of processing a sublist,
group, or indirect specification. The effect of enabling or disabling
backtracking is limited to the remainder of the level currently being
processed and lower levels.
Backtracking is disabled while matching any of the
form specifications (that is, form, body, def-form, and
def-body). These specifications will match any form so any error
must be in the form itself rather than at a higher level.
Backtracking is also disabled after successfully matching a quoted
symbol, string specification, or &define keyword, since this
usually indicates a recognized construct. But if you have a set of
alternative constructs that all begin with the same symbol, you can
usually work around this constraint by factoring the symbol out of the
alternatives, e.g., ["foo" &or [first case] [second case] ...].
Most needs are satisfied by these two ways that backtracking is
automatically disabled, but occasionally it is useful to explicitly
disable backtracking by using the gate specification. This is
useful when you know that no higher alternatives could apply. See the
example of the let specification.