code: func Reduce(expr Expr, valuer Valuer) Expr {
  	expr = reduce(expr, valuer)
  
  	// Unwrap parens at top level.
  	if expr, ok := expr.(*ParenExpr); ok {
  		return expr.Expr
  	}
  	return expr
  }
docs: // Reduce evaluates expr using the available values in valuer.
  // References that don't exist in valuer are ignored.