isKey

// Check less_than
   import std.conv : to;

   assert(less_than("a", "b"));
   assert(less_than(0.to!string, 1.to!string));
   assert(!less_than("00", "0"));
   assert(less_than("0", "abe"));

   assert(less_than("7", "69"));
   // assert(less_than(0, "1"));
   // assert(less_than(5, 7));