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(Fadd_text_properties, Fremove_text_properties):

Add len>0 as condition for main loop.
Abort if reach a null interval.
(Fset_text_properties): Abort if reach a null interval.
(Ftext_properties_at, Fget_text_property):
Return nil if POS is end of OBJECT.
(add_properties): Use NILP to test result of Fequal.
No longer inline.
(remove_properties): No longer inline.
(set_properties): Total rewrite as function.
(validate_interval_range): Don't alter *begin at end of buffer.
But do search for a position just before the end.
Return null for an empty string.

(validate_interval_range): Allow 0 as position in string.
Add 1 to specified string positions.
(Fprevious_single_property_change): Subtract 1 if object is string.
(Fnext_single_property_change): Likewise.
(Fprevious_property_change, Fnext_property_change): Likewise.

(remove_properties): Call modify_buffer.
(add_properties): Likewise.

(Fadd_text_properties): Pass new arg to add_properties.
(Fremove_text_properties): Likewise.
(add_properties, remove_properties): New arg OBJECT.  Record undo info.
(Fput_text_property): New function.