feat: nil/null/empty/blank literals, resolves #102

src/drop/null-drop.ts

@@ -0,0 +1,26 @@
+import { Drop } from './drop'
+import { IComparable } from './icomparable'
+import { isNil } from 'src/util/underscore'
+import { IDrop, isDrop } from 'src/drop/idrop'
+import { BlankDrop } from 'src/drop/blank-drop'
+
+export class NullDrop extends Drop implements IDrop, IComparable {
+  equals (value: any) {
+    return isNil(isDrop(value) ? value.value() : value) || value instanceof BlankDrop
+  }
+  gt () {
+    return false
+  }
+  geq () {
+    return false
+  }
+  lt () {
+    return false
+  }
+  leq () {
+    return false
+  }
+  value () {
+    return null
+  }
+}