code: func (p *parser) parseExpression(tok *lexer.Token) *ast.Expression {
  	result := ast.NewExpression(tok.Pos, tok.Line)
  
  	// helperName
  	result.Path = p.parseHelperName()
  
  	// param* hash?
  	result.Params, result.Hash = p.parseExpressionParamsHash()
  
  	return result
  }
docs: // helperName param* hash?