code: @Nonnull
      private Expression buildExpression( String str ) {
          switch( str.charAt( 0 ) ) {
              case '@':
                  return new VariableExpression( reader, str );
              case '-':
                  if( str.startsWith( "-@" ) ) {
                      return new FunctionExpression( reader, "-", new Operation( reader, buildExpression( str.substring( 1 ) ), (char)0 ) );
                  }
                  //$FALL-THROUGH$
              default:
                  return new ValueExpression( reader, str );
          }
      }
docs: Create an expression from the given atomic string.
  
  @param str the expression like a number, color, variable, string, etc
  @return the expression