graph  		:  	[ strict ] (graph | digraph) [ ID ] '{' stmt_list '}'
stmt_list 	: 	[ stmt [ ';' ] [ stmt_list ] ]
stmt 		: 	node_stmt
		| 	edge_stmt
		| 	attr_stmt
		| 	ID '=' ID
		| 	subgraph
attr_stmt 	: 	(graph | node | edge) attr_list
attr_list 	: 	'[' [ a_list ] ']' [ attr_list ]
a_list 		: 	ID [ '=' ID ] [ ',' ] [ a_list ]
edge_stmt 	: 	(node_id | subgraph) edgeRHS [ attr_list ]
edgeRHS 	: 	edgeop (node_id | subgraph) [ edgeRHS ]
node_stmt 	: 	node_id [ attr_list ]
node_id 	: 	ID [ port ]
port 		: 	':' ID [ ':' compass_pt ] | ':' compass_pt
subgraph 	: 	[ subgraph [ ID ] ] '{' stmt_list '}'
compass_pt 	: 	(n | ne | e | se | s | sw | w | nw | c | _)
ID		: [-]?(.[0-9]+ | [0-9]+(.[0-9]*)? | [a-zA-Z\200-\377]
//extension
drop_graph	: drop ID
constraint_list	: '(' ID ',' ID ')' [constraint_list]
add_vertices	: add vertices ID '{' node_stmt '}'
remove_vertices	: remove vertices ID '{' a_list '}'
add_edges	: add edges ID '{' edge_stmt '}'
remove_edges	: remove edges ID '{' edge_stmt '}'
set_vertex_constraints	: set vertices constraint ID '{' a_list '}'	
set_order_constraints	: set order constraint ID '{' constraint_list '}'