R/attributes-concept.R
valueAsString.RdSerializes to ValueAsString: { Text, Op }; Circe generates WHERE value_as_string LIKE ....
valueAsString(text, op = "contains")Character string to match (e.g. "Yes").
Circe op: "contains" (default, LIKE %text%), "starts", "ends", "equals".
Attribute for use in observation().