Creating a ://: repeat mark?

[Matthew Vernon]

Hi,

Sometimes music has marks a little like ://: (only with the
double-dots slanted too) to indicate that a phrase of the words is to
be repeated at this point in the music. How can I get lilypond to
generate such a mark?

[An example would be in the hymn tune "Cwm Rhondda", to show that the
final line of each verse is to be sung twice]

Thanks,

Matthew

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