Elaborate on declarations

spec/04-declarations.md

@@ -1,9 +1,35 @@
 # Chapter 4: Declarations
 
-Declarations establish a binding between a name and a value, or sometimes
-several such name/value bindings. Unlike expressions whose primary role is to
-be evaluated, and statements which are primarily executed, declarations add
-their binding to the lexical environment.
+Declarations establish a binding between a name and a value. Unlike expressions
+whose primary role is to be evaluated, and statements which are primarily
+executed, declarations add their binding to the lexical environment.
+
+A declaration belongs to its innermost enclosing block. As a block is run,
+first a new frame is created, empty but linked to the frame of the innermost
+surrounding block, if any. All the declarations belonging to the block get a
+binding with their name, to a pseudo-value called `uninitialized`. After that,
+declarations initialize on two different schedules, depending on their type.
+
+_Hoisted declarations_ initialize immediately, meaning that their binding will
+be initialized by the time the first statement is executed. Hoisted
+declarations of the same block are free to mutually refer to each other, since
+by design their initialization doesn't evaluate any expressions or execute any
+statements; when they do refer to each other, they do so in code which is not
+yet running. The specific initialization order of hoisted declarations in a
+block is unobservable; any order produces the same end result.
+
+_Inline declarations_ (only variable declarations in the base language)
+have an expression called an _initialized expression_, which is evaluated the
+moment execution reaches the declaration. The resulting value is then used to
+initialize the declared name. Reading a variable before it has been initialized
+(which includes in the initializer expression itself) results in a runtime
+error.
+
+Many declaration forms are _complex_ and contain nested declarations. These
+nested declarations are typically for a different scope. For example, function
+declarations contain parameters, which declare names to be used in the function
+body. Classes declare fields or methods, which declare names to be used in
+connection with instances of the class.
 
 ```
 <declaration> ::= <variable-declaration>