sequence membership and sequence builder operators

as you now for sets there are
- a set membership operator, ∈
- a set builder notation { | } (usage example: {n ∈ N | n^2 < 20})

see e.g. Table of mathematical symbols - Wikipedia, the free encyclopedia

which are the correspondent operators to use for sequences (Sequence - Wikipedia, the free encyclopedia)?
I need a sequence membership operator and a sequence builder notation.

Thanks,
Mauro