Function builtin_substr 

fn builtin_substr(
    args: &[RValue],
    _: &[(String, RValue)],
) -> Result<RValue, RError>

Extract substrings from character strings.

Vectorized over x, start, and stop with recycling.

@param x character vector to extract from @param start integer vector of starting positions (1-indexed) @param stop integer vector of ending positions (inclusive) @return character vector containing the substrings