PT[101]% bison -y -d ch3-03.y
PT[102]% flex ch3-03.l
PT[103]% gcc lex.yy.c y.tab.c -ll -ly

PT[104]% ./a.out
a =4
4 * a
= 16
b = 7
a * (b + -2)
= 20
a /2
= 2
b / 2
= 3.5
^D
PT[105]%