Check text from START to END for property PROPERTY not equaling VALUE.
If so, return the position of the first character whose property PROPERTY
is not `eq' to VALUE. Otherwise, return nil.
If the optional fifth argument OBJECT is a buffer (or nil, which means
the current buffer), START and END are buffer positions (integers or
markers). If OBJECT is a string, START and END are 0-based indices into it.
(fn START END PROPERTY VALUE &optional OBJECT)