This function has two issues: it is very slow and works not as expected (in a sence that it takes two seqs).
For a haskell-like filter function (which runs quite fast) use `hfilter'.
hfilter[predicate-function; sequence-to-filter]
filter/2: Filter out corresponding elements to 0
filter[filter-sequence; sequence-to-filter]
hfilter: {filter[y map x; x]}