def aggregate_field(self, field_name, rows):
field = self.fields.get(field_name,{})<EOL>if "<STR_LIT>" not in field:<EOL><INDENT>return rows[-<NUM_LIT:1>][field_name]<EOL><DEDENT>aggregate = getattr(self,<EOL>"<STR_LIT>".format(field.get("<STR_LIT>")))<EOL>r = aggregate(field_name, rows)<EOL>return r<EOL>
takes a field name and a set of rows and will
return an aggregate value

this requires for the field to have it's `aggregate`
config specified in the probe config