Evaluating the expression in brackets first,$$(1 
abla 2) 
abla 3 =\left( \frac{1 + 2}{1 + 1 \times 2}\right) 
abla 3 = \left(\frac{3}{3}\right) 
abla 3 = 1 
abla 3 = \frac{1 + 3}{1 + 1 \times 3} = 1.$$Note that for any $b>0,$ $$1
abla b =\frac{1+b}{1+1\times b}=\frac{1+b}{1+b}=1.$$
If $a>0$ and $b>0,$ a new operation $
abla$ is defined as follows:  $$a 
abla b = \frac{a + b}{1 + ab}.$$For example, $$3 
abla 6 = \frac{3 + 6}{1 + 3 \times 6} = \frac{9}{19}.$$Calculate $(1 
abla 2) 
abla 3.$