code: function warnIfGridRowColumnExists (decl, result) {
    let rule = decl.parent
    let decls = []
    rule.walkDecls(/^grid-(row|column)/, d => {
      if (!/-end$/.test(d.prop) && !/^span/.test(d.value)) {
        decls.push(d)
      }
    })
    if (decls.length > 0) {
      decls.forEach(d => {
        d.warn(result,
          `You already have a grid-area declaration present in the rule. ` +
          `You should use either grid-area or ${ d.prop }, not both`)
      })
    }
  
    return undefined
  }
docs: warn user if both grid-area and grid-(row|column)
  declarations are present in the same rule
  @param  {Declaration} decl
  @param  {Result} result
  @return {void}