code: func Expr(str string) (node ast.Node, err error) {
  	var t = &tree{lex: lexExpr("", str)}
  	defer t.recover(&err)
  	return t.parseExpr(0), err
  }
docs: // Expressions ----------
  // Expr returns the parsed representation of the given soy expression.
  // An expression is basically anything that you can put inside a print tag.
  // For example, string, list or map literals, arithmetic, boolean operations, etc.