+ import std.math;
  import std.typecons;
- import std.math;
  /*
   Input are two strings a and b consisting only of 1s and 0s.
-     Perform binary XOR on these inputs and return result also as a string.
? ----

+ Perform binary XOR on these inputs and return result also as a string.
-     >>> string_xor("010", "110")
? ----

+ >>> string_xor("010", "110")
+ "100"
-     "100"
-     
  */
  string string_xor(string a, string b)