text immediately before point. Returning nil from this command passes
control to the next backend. The function should return `stop' if it
should complete but cannot (e.g. if it is in the middle of a string).
-Instead of a string, the backend may return a cons where car is the prefix
-and cdr is used instead of the actual prefix length in the comparison
-against `company-minimum-prefix-length'. It must be either number or t,
-and in the latter case the test automatically succeeds.
+Instead of a string, the backend may return a cons (PREFIX . LENGTH)
+where LENGTH is a number used in place of PREFIX's length when
+comparing against `company-minimum-prefix-length'. LENGTH can also
+be just t, and in the latter case the test automatically succeeds.
`candidates': The second argument is the prefix to be completed. The
return value should be a list of candidates that match the prefix.