This closes #1319, fix calculate error for formula with negative symbol

- Update unit test and comment for the functions

Author: xuri

calc.go

@@ -1234,7 +1234,7 @@ func calculate(opdStack *Stack, opt efp.Token) error {
 			return ErrInvalidFormula
 		}
 		opd := opdStack.Pop().(formulaArg)
-		opdStack.Push(newNumberFormulaArg(0 - opd.Number))
+		opdStack.Push(newNumberFormulaArg(0 - opd.ToNumber().Number))
 	}
 	if opt.TValue == "-" && opt.TType == efp.TokenTypeOperatorInfix {
 		if opdStack.Len() < 2 {
@@ -1647,18 +1647,18 @@ func formulaCriteriaEval(val string, criteria *formulaCriteria) (result bool, er
 	var value, expected float64
 	var e error
 	prepareValue := func(val, cond string) (value float64, expected float64, err error) {
-		percential := 1.0
+		percentile := 1.0
 		if strings.HasSuffix(cond, "%") {
 			cond = strings.TrimSuffix(cond, "%")
-			percential /= 100
+			percentile /= 100
 		}
 		if value, err = strconv.ParseFloat(val, 64); err != nil {
 			return
 		}
 		if expected, err = strconv.ParseFloat(cond, 64); err != nil {
 			return
 		}
-		expected *= percential
+		expected *= percentile
 		return
 	}
 	switch criteria.Type {

calc_test.go

@@ -491,6 +491,7 @@ func TestCalcCellValue(t *testing.T) {
 		// COS
 		"=COS(0.785398163)": "0.707106781467586",
 		"=COS(0)":           "1",
+		"=-COS(0)":          "-1",
 		"=COS(COS(0))":      "0.54030230586814",
 		// COSH
 		"=COSH(0)":       "1",

comment.go

@@ -177,7 +177,7 @@ func (f *File) addDrawingVML(commentID int, drawingVML, cell string, lineCount,
 				},
 			},
 		}
-		// load exist comment shapes from xl/drawings/vmlDrawing%d.vml (only once)
+		// load exist comment shapes from xl/drawings/vmlDrawing%d.vml
 		d := f.decodeVMLDrawingReader(drawingVML)
 		if d != nil {
 			for _, v := range d.Shape {

rows.go

@@ -179,6 +179,7 @@ func (rows *Rows) Columns(opts ...Options) ([]string, error) {
 	return rowIterator.columns, rowIterator.err
 }
 
+// extractRowOpts extract row element attributes.
 func extractRowOpts(attrs []xml.Attr) RowOpts {
 	rowOpts := RowOpts{Height: defaultRowHeight}
 	if styleID, err := attrValToInt("s", attrs); err == nil && styleID > 0 && styleID < MaxCellStyles {