From type/independent-routines

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multi sub undefine(Mu    \x)
multi sub undefine(Array \x)
multi sub undefine(Hash  \x)

DEPRECATED in 6.d language version and will be removed in 6.e. For Array and Hash, it will become equivalent to assigning Empty; for everything else, equivalent to assigning Nil or Empty in the case of arrays or hashes, whose use is advised.