@property<EOL><INDENT>def start(self):<DEDENT>
if self.item is None:<EOL><INDENT>return self._start<EOL><DEDENT>else:<EOL><INDENT>if self._start != self.item.start:<EOL><INDENT>raise IndexError("<STR_LIT>" +<EOL>"<STR_LIT>")<EOL><DEDENT>return self.item.start<EOL><DEDENT>
:return: the genomic start of this alignment block in ref seq.