Test_2.scala:2: error: cto may only be used as an argument to m
  M.cto // error
    ^
Test_2.scala:3: error: cto may only be used as an argument to m
  M.m(M.cto, ()) // error
        ^
Test_2.scala:5: error: cto may only be used as an argument to m
  M.cto // error
    ^
three errors found