public TableBuilder <TDoc, T> WithColumn <R>(string header, Func <T, Maybe <R> > selector, Writer <TDoc, Maybe <R> > writer)
 {
     return(WithColumn(Writers <TDoc> .Const(header), selector, writer));
 }
 public TableBuilder <TDoc, T> WithColumn <R>(string header, Func <T, R> selector)
 {
     return(WithColumn(Writers <TDoc> .Const(header), selector, Writers <TDoc> .Cell <R>()));
 }