code: public String addMapping(String prefix, String uri)
      {
          String[] strs = mNsStrings;
          int phash = prefix.hashCode();
  
          for (int ix = mScopeStart, end = mScopeEnd; ix < end; ix += 2) {
              String thisP = strs[ix];
              if (thisP == prefix ||
                  (thisP.hashCode() == phash && thisP.equals(prefix))) {
                  // Overriding an existing mapping
                  String old = strs[ix+1];
                  strs[ix+1] = uri;
                  return old;
              }
          }
          // no previous binding, let's just add it at the end
          if (mScopeEnd >= strs.length) {
              // let's just double the array sizes...
              strs = DataUtil.growArrayBy(strs, strs.length);
              mNsStrings = strs;
          }
          strs[mScopeEnd++] = prefix;
          strs[mScopeEnd++] = uri;
  
          return null;
      }
docs: Method to add a new prefix-to-URI mapping for the current scope.
  Note that it should NOT be used for the default namespace
  declaration
  
  @param prefix Prefix to bind
  @param uri URI to bind to the prefix
  
  @return If the prefix was already bound, the URI it was bound to:
  null if it's a new binding for the current scope.