def size(self, pairs_only=False):
if pairs_only:<EOL><INDENT>return len(self.just_pairs)<EOL><DEDENT>else:<EOL><INDENT>return len(self.heavies)<EOL><DEDENT>
Calculate the size of the lineage.

Inputs (optional)
-----------------
pairs_only: count only paired sequences

Returns
-------
Lineage size (int)