def process(self, key, val):
for field in self.fields:<EOL><INDENT>if field.check(key, val):<EOL><INDENT>return<EOL><DEDENT><DEDENT>for field in self.optional:<EOL><INDENT>if field.check(key, val):<EOL><INDENT>return<EOL><DEDENT><DEDENT>
Try to look for `key` in all required and optional fields. If found,
set the `val`.