First it looks to see whether there is a quote before the list; if there is, the interpreter
just gives us the list.
On the other hand, if there is no quote, the interpreter looks at the first element in the
list and sees whether it has a function definition. If it does, the interpreter carries out
the instructions in the function definition. Otherwise, the interpreter prints an error
message.
原文：http://www.cnblogs.com/aqing1987/p/4203730.html