Function interp_unsplit 

fn interp_unsplit(
    positional: &[RValue],
    _named: &[(String, RValue)],
    _context: &BuiltinContext<'_>,
) -> Result<RValue, RError>

Reverse of split: reassemble a vector from a split list.

@param value list of vectors (as produced by split()) @param f factor or vector defining the groups (same length as the original vector) @return vector with elements placed back at their original positions