code: func (p *parser) parseSexpr() *ast.SubExpression {
  	// OPEN_SEXPR
  	tok := p.shift()
  
  	result := ast.NewSubExpression(tok.Pos, tok.Line)
  
  	// helperName param* hash?
  	result.Expression = p.parseExpression(tok)
  
  	// CLOSE_SEXPR
  	tok = p.shift()
  	if tok.Kind != lexer.TokenCloseSexpr {
  		errExpected(lexer.TokenCloseSexpr, tok)
  	}
  
  	return result
  }
docs: // sexpr : OPEN_SEXPR helperName param* hash? CLOSE_SEXPR