Add '19.nix'

19.nix

@@ -0,0 +1,21 @@
+let
+  attrSet = {x = "a"; y = "b"; b = {t = true; f = false;};};
+  attrSet.c = 1;
+  attrSet.d = null;
+  attrSet.e.f = "g";
+in
+rec {
+  #boolean
+  ex0 = attrSet.b.t;
+  #equal
+  ex01 =  "a" == attrSet.x; 
+  #unequal 
+  ex02 = !("b" != attrSet.y );
+  #and/or/neg
+  ex03 = ex01 && !ex02 || ! attrSet.b.f;
+  #implication
+  ex04 = true -> attrSet.b.t;
+  #contains attribute
+  ex05 = attrSet ? x;
+  ex06 = attrSet.b ? f;
+}