WaterDoc: thing.execute_expr - a defmethod

Contract	a_thing.<execute_expr env_or_context=null execution_kind="ek_code" returns="last" />
See also	docs

Takes an expression as "_subject", executes it and returns the result. foo. /> value. /> executes each expr in a_vec and returns the last one. The env_or_context parameter is documented in execute_string.