subseq returns a copy of subsequence of string or vector str using zero-based
    indices from (inclusive) and to (exclusive) to delimit the substring or the vector
    extracted.

    If to is omitted or is NULL, then it equals by default to the length of
    str, i.e. everything from from to the end of str is returned.

    If to and from are equal, an empty string @@(empty vector) is returned.

    If from is greater than to or length of str an error is signalled.

    If str is NULL then NULL is returned.

The last one with string argument returns a copy of the string cut from the first slash,
leaving it and everything following out, and in the case where there
are no slashes present, returns a copy of the whole string.
