Add precision getter to NaiveNumber.

src/org/nwapw/abacus/number/NaiveNumber.java

@@ -8,6 +8,11 @@ public class NaiveNumber implements Number {
         this.value = value;
     }
 
+    @Override
+    public int precision() {
+        return 4;
+    }
+
     @Override
     public Number multiply(Number multiplier) {
         if(!(multiplier instanceof NaiveNumber)) throw new IllegalArgumentException();

src/org/nwapw/abacus/number/Number.java

@@ -2,6 +2,7 @@ package org.nwapw.abacus.number;
 
 public interface Number {
 
+    int precision();
     Number multiply(Number multiplier);
     Number divide(Number divisor);
     Number add(Number summand);