Add filter modifiers

[pokey]

Returns [] if input passes filter, otherwise [inputTarget]

• "empty": keepEmptyFilter zero length or whitespace content only
• "content" keepContentFilter nonzero length and nonwhitespace content (=!keepEmptyFilter)
• Don't use "whitespace"

Can be used for eg "take content every line"

Other notes

Filter viewpoint

• is it nonzero length?
• does it have visible characters selected?

empty
empty and not adjacent to visible characters
empty and adjacent to visible characters
nonempty
nonempty and only containing whitespace
nonempty and containing visible chars