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Velocity Specification - AST
> Please note that this is currently out of date and will be updated soon.
>     This is tentatively the AST structure for Velocity.

>     The structure is as would be described using ANTLR
>     BNF-style notation.  A tree is described by
>     #(node node node) where #() represents the tree form.
>     The first node is the root of the tree, and following
>     nodes are all children.

>     Visually, #(A B C D E) might look like:

>          A
>          |
>          B-C-D-E

>     Root nodes must always be a terminal node, denoted by
>     an all-caps identifier.

>     Non-root nodes may be either terminal nodes, or sub-rules,
>     or inlined sub-trees.

>     A sub-rule might describe a tree, or simple a node.

>     block
>     	:
>     		#(	BLOCK
>     				(statement)*
>     		)

>     	;
>     statement
>     	:
>     			text
>     		|	if_statement
>     		|	foreach_statement
>     		|	include_statement
>     		|	set_statement
>     		|	parse_statement
>     		|	param_statement
>     		|	stop_statement
>     		|	reference
>     	;

>     text
>     	:
>     		TEXT
>     	;

>     //	if/elseif/else chains should be represented solely
>     //	by <if_statement>.  <elseif> is simply an <if_statement>
>     //	in the false-branch of the preceeding <if_statement>
>     //	while <else> is simply an <if_statement> where the
>     //	<expr> evaluates to TRUE always, and no false-branch
>     //	is provided.

>     if_statement
>     	:
>     		#(	IF
>     				// The expression to test
>     				expr

>     				// True branch
>     				block

>     				// False branch
>     				( block )?
>     		)
>     	;

>     foreach_statement
>     	:
>     		#(	FOREACH
>     				// Value to assign for each iteration
>     				reference
>     				// List of objects to iterator
>     				reference
>     				// command-block to execute
>     				( block )?
>     		)
>     	;

>     set_statement
>     	:
>     		#(	SET
>     				// Variable to set
>     				reference
>     				// Value to assign
>     				expr
>     	;

>     parse_statement
>     	:
>     		#(	PARSE
>     				STRING_LITERAL
>     		)
>     	;

>     include_statement
>     	:
>     		#(	INCLUDE
>     				STRING_LITERAL
>     		)
>     	;

>     stop_statement
>     	:
>     		STOP
>     	;

>     reference
>     	:
>     		#(	REFERENCE
>     				postfix
>     	;

>     postfix
>     	:
>     		method_call
>     		|
>     		member
>     		|
>     		identifier
>     	;

>     member
>     	:
>     		#(	DOT
>     				identifier
>     				(	primary
>     				|	method_call
>     				)
>     	;

>     primary
>     	:
>     		IDENTIFIER
>     	;

>     method_call
>     		#(	CALL
>     				postfix
>     				// The argument list
>     				( expr ) *
>     		)
>     	;
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