code: func NewExpression(s string) *Graph {
  	p := NewBytesParser([]byte(s))
  	p.Expression()
  	g := p.Graph()
  	g.This = TypeExpression
  	g._ast()
  
  	return g
  }
docs: // NewExpression parses an expression in text format (given in the string) to a Graph,
  // in the form of a suitable syntax tree.
  //
  //     expression := expr1 (op2 expr1)*
  //     expr1 := path | constant | op1 path | op1 constant | '(' expr ')' | op1 '(' expr ')'
  //     constant ::= quoted | number