Update 'Variable Definition'

Variable-Definition.md

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+In ARF, global variables are defined using data sections. However, local variables are defined whenever there is a declaration. Declarations consist of a variable name, a colon, and a type specifier.
+
+```
+x:Int
+
+x:Int:<5>
+
+x:Obj:(
+	.pv this:Int
+	.pv that:Int)
+```
+
+Everything follows a pattern of definition -> instantiation. Variables are defined by defining their type and initial value using a type specifier, and they are instantiated at that point in the code. Wheverever in a phrase a variable can be used, it can be defined.
+
+It is very useful to define a variable's value based on the return value from a function call. For this purpose, [return direction](Return-Direction) is used.
+
+To define a variable normally, a `let` phrase is used.
+
+```
+let x:Int:<5>
+```
+
+Multiple variables may be defined in a single `let` phrase.
+
+```
+let x:Int y:Int z:Int
+```
+
+You can even define a variable as an argument to a function.
+
+```
+file.read buffer:Byte:16 -> amountRead:Size error:Error
+```