anchor.diff:        xf(borderId="0" fillId="0" fontId="0" numFmtId="0")/
       cellXfs(count=cellStyles.length)
         each style, t in cellStyles
  +        - var fontId = ('font' in style && style.font) || 0
           - var fillId = ('fill' in style && style.fill) || 0
           - var borderId = ('border' in style && style.border) || 0
           - var numberFormat = ('numberFormat' in style && (style.numberFormat + 200)) || 0
  -        if Object.keys(style).length === 1
  +        if !style || Object.keys(style).length === 1
             xf/
           else
             xf(
  
positive.code: extends ./xml.jade
  include ../mixins/attrTags.jade
  
  mixin valTag(tag, val)
    - var tagMap = {size: 'sz'}
  
negative.code:         fill
            if (fill.type === 'pattern')
              patternFill(patternType=fill.pattern||'solid')
                if fill.pattern === 'solid' || fill.color
                  fgColor/&attributes(fill.fgColor || fill.color)
                if fill.bgColor
                  bgColor/&attributes(fill.bgColor)
  