PT[101]% bison -y -d ch3-02.y
PT[102]% flex ch3-01.l
PT[103]% gcc lex.yy.c y.tab.c -ll -ly

PT[104]% ./a.out
3 + 4 * 5
= 23
PT[105]% ./a.out
3 + 4 * -5
= -17
PT[106]% ./a.out
3 - 4 + 5
= 4
PT[107]% ./a.out
3 - -4 + 5
= 12
PT[108]% ./a.out
5 / 2
= 2
PT[109]%