/* package whatever; // don't place package name! */
 
import java.util.function.BinaryOperator;
import java.util.*;
import java.lang.*;
import java.io.*;
 
/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		BinaryOperator<String>l=(m,t)->m.contains(t)?m.replace(t,""):m+t;
        System.out.println(l.apply("", "a"));
        System.out.println(l.apply("a", "a"));
        System.out.println(l.apply("b", "a"));
        System.out.println(l.apply("ab", "a"));
        System.out.println(l.apply("aba", "a"));
        System.out.println(l.apply("ababa", "aba"));
	}
}

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