NullSink

import std.algorithm.iteration : map;
import std.algorithm.mutation : copy;
[4, 5, 6].map!(x => x * 2).copy(nullSink); // data is discarded

import std.csv : csvNextToken;

string line = "a,b,c";

// ignore the first column
line.csvNextToken(nullSink, ',', '"');
line.popFront;

// look at the second column
Appender!string app;
line.csvNextToken(app, ',', '"');
assert(app.data == "b");