code: @Nonnull
    public EChange add (@Nonnull final Map.Entry <String, String> aEntry)
    {
      return add (aEntry.getKey (), aEntry.getValue ());
    }
docs: Add a new variable.
  
  @param aEntry
  The entry to be added - key is the variable name and value is the
  variable value. May not be <code>null</code>.
  @return {@link EChange#UNCHANGED} if a variable with the same name is
  already present. Never <code>null</code>.