Slur across voices without taking time with hidden notes

[Nick Payne]

In this example, excerpted from a score, how can I get a slur between the
two notes? The snippets repository shows slurs across voices using hidden
notes, but the hidden notes are counted in the timing.

\relative c' {
        \time 3/4
        << {
                b'4\rest e,2 |
        } \\ {
                a,2.
        } >>
}

Nick Payne