public static Unit ifLeft <L, R>(Either <L, R> either, Action <L> Left) => either.IfLeft(Left);
public static R ifLeft <L, R>(Either <L, R> either, Func <L, R> leftMap) => either.IfLeft(leftMap);
public static R ifLeft <L, R>(Either <L, R> either, R rightValue) => either.IfLeft(rightValue);
public static R ifLeft <L, R>(Either <L, R> either, Func <R> Left) => either.IfLeft(Left);