(期末了,该预习编译原理了2333)

Colloquially, we say that "finite automata cannot count," meaning that
a finite automaton cannot accept a language like {anbnn1}\{a^nb^n|n\ge1\} that would
require it to keep count of the number of a ' s before it sees the b's.
Likewise, "a grammar can count two items but not three," as we shall see when we consider non-context-free language constructs in Section 4.3.5.

a grammar can not count three items:
a non-CFG: {anbncnn1}\{a^nb^nc^n|n\ge1\}