add examples

examples/stacktricks.js

@@ -0,0 +1,17 @@
+
+stack("c","e","g")
+  
+    .withHap((hap)=>{ 
+        hap.context.tempvalue=hap.value;
+        hap.value=hap.context.stacking===undefined?0:hap.context.stacking[0];
+        return hap;})
+    
+    .eq("<0 1 2>").filterValues((val) => val)
+    
+    .withHap((hap)=>{ 
+        hap.value=hap.context.tempvalue;
+        delete hap.tempvalue;
+        return hap;})
+
+  .note().piano()
+