[gkop]

It was proposed in 2010 and rejected by Matz in 2012: https://bugs.ruby-lang.org/issues/3845

[singularity2001]

something went wrong between

Matz: I am neutral for this proposal.

... ...

Matz: This proposal is only for cosmetics. I don't want a new operator that does not introduce something new.

[I personally disagree and think the 'in' operator should be added]