Variable Attributes

Any variable may have one of the following attributes:

`const`	the variable may not be modified by the program, i.e. it is not allowed on the left-hand side of an assignment statement.
`volatile`	warns the compiler that the variable might be altered by some process external to the program.
`extern`	the variable is external, declared as a global variable in another file.
`static`	(on a global variable) makes the variable local to this file, i.e. it cannot be referred to in another file using the `extern` attribute.
	(on a local variable) allocates permanent storage for the variable, i.e. it is only initialised once, and it preserves its value between function calls.
`auto`	(the default for local variables) means that temporary storage is allocated for the variable on the stack, and its value is lost when the function returns.
`register`	if possible, the compiler will store the variable in a processor register rather than on the stack.