presto-main/src/main/java/io/prestosql/operator/window/LeadFunction.java/LeadFunction/LeadFunction
class LeadFunction:    public LeadFunction(List<Integer> argumentChannels, boolean ignoreNulls)
    {
        this.valueChannel = argumentChannels.get(0);
        this.offsetChannel = (argumentChannels.size() > 1) ? argumentChannels.get(1) : -1;
        this.defaultChannel = (argumentChannels.size() > 2) ? argumentChannels.get(2) : -1;
        this.ignoreNulls = ignoreNulls;
    }