+=item mtime_ischanged
+
+Tries to avoid concurrent write operations to records by checking the database mtime with a fetched one.
+
+Can be used / called with any table, that has itime and mtime attributes.
+Valid C<table> names are: oe, ar, ap, delivery_orders, parts.
+Can be called wit C<option> mail to generate a different error message.
+
+Returns undef if no save operation has been done yet ($self->{id} not present).
+Returns undef if no concurrent write process is detected otherwise a error message.
+