\version "2.21.1"

cantus = {
  \time 2/4
  a4 b(\grace{a16 b c')} a2
  a4 b(\afterGrace{a16 b c')} a2
  b2
}

\score {
    \context Staff = "one" <<
      \new Voice = "cantus" {
        \cantus
      }
    >>
  }