Consider the following grammar:
stmt $\style{font-family:'Times New Roman'}\rightarrow$ if expr then expr else expr; stmt | 0
expr $\style{font-family:'Times New Roman'}\rightarrow$ term relop term | term
term $\style{font-family:'Times New Roman'}\rightarrow$ id |number
id $\style{font-family:'Times New Roman'}\rightarrow$ a | b | c
number $\style{font-family:'Times New Roman'}\rightarrow$ [0-9]
where relop is a rational operator (e.g.,<,>,...), O refers to the empty statement, and if, then, else are terminals.
Consider a preogram P following the above grammar containing ten if terminals. The number of control flow paths in P is ________. For example, the program
if e_{1} then e_{2} else e_{3}
has 2 control flow paths, $\style{font-family:'Courier New'}{\begin{array}{l}{\mathrm e}_1\;\rightarrow\;{\mathrm e}_2\;\mathrm{and}\;{\mathrm e}_1\;\rightarrow\;{\mathrm e}_3\\\;\end{array}}$.