以下摘自The Go Programming Language：
A syntactic block is a sequence of statements enclosed in braces like those that surround the body of a function or loop. A name declared inside a syntactic block is not visible outside that block. The block encloses its declarations and determines their scope. We can generalize this notion of blocks to include other groupings of declarations that are not explicitly surrounded by braces in the source code; we’ll call them all lexical blocks. There is a lexical block for the entire source code, called the universe block; for each package; for each file; for each for, if, and switch statement; for each case in a switch or select statement; and, of course, for each explicit syntactic block.
A declaration’s lexical block determines its scope, which may be large or small. The declarations of built-in types, functions, and constants like int, len, and true are in the universe block and can be referred to throughout the entire program. Declarations outside any function, that is, at package level, can be referred to from any file in the same package. Imported packages, are declared at the file level, so they can be referred to from the same file, but not from another file in the same package without another import. Many declarations, like that of the variables in the function, are local, so they can be referred to only from within the same function or perhaps just a part of it.
The scope of a control-flow label, as used by break, continue, and goto statements, is the entire enclosing function.
需要注意universe block的定义：Go语言的built-in类型，函数和常量的scope都属于universe block。