( Variables; syntactic keywords; and regions

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 3.1 Variables; syntactic keywords; and regions
 An identifier may name a type of syntax, or it may name a location
 where a value can be stored.  An identifier that names a type of syntax
 is called a _syntactic keyword_ and is said to be _bound_ to that
 syntax.  An identifier that names a location is called a _variable_ and
 is said to be _bound_ to that location.  The set of all visible
 bindings in effect at some point in a program is known as the
 _environment_ in effect at that point.  The value stored in the
 location to which a variable is bound is called the variable's value.
 By abuse of terminology, the variable is sometimes said to name the
 value or to be bound to the value.  This is not quite accurate, but
 confusion rarely results from this practice.
 Certain expression types are used to create new kinds of syntax and
 bind syntactic keywords to those new syntaxes, while other expression
 types create new locations and bind variables to those locations.
 These expression types are called _binding constructs_.
 Those that bind syntactic keywords are listed in section  Macros.
 The most fundamental of the variable binding constructs is the `lambda'
 expression, because all other variable binding constructs can be
 explained in terms of `lambda' expressions.  The other variable binding
 constructs are `let', `let*', `letrec', and `do' expressions (see
 sections  Procedures,  Binding constructs, and 
 Like Algol and Pascal, and unlike most other dialects of Lisp except
 for Common Lisp, Scheme is a statically scoped language with block
 structure.  To each place where an identifier is bound in a program
 there corresponds a "region" of the program text within which the
 binding is visible.  The region is determined by the particular binding
 construct that establishes the binding; if the binding is established
 by a `lambda' expression, for example, then its region is the entire
 `lambda' expression.  Every mention of an identifier refers to the
 binding of the identifier that established the innermost of the regions
 containing the use.  If there is no binding of the identifier whose
 region contains the use, then the use refers to the binding for the
 Expressions:: and  Standard procedures); if there is no binding
 for the identifier, it is said to be "unbound".  
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