Issue on page /markdown/chp_11.html (imprecise example of 0 probability) #210

One of the example in Section 11.1.1 is slightly imprecise, since $P(A)$ is defined only over subsets of $\mathcal{X}$. Therefore $P(A=-1)$ is undefined, a better example (in fact the only one!) is $P(A=\lbrace\rbrace) = 0$