title: Using xlsxwriter merge_row errors
body: <p>I am exporting an excel file with xlsxwriter and I am trying to merge multiple cells using <code>.merge_range()</code> but I appear to be receiving errors related to constant memory even though I have set this to False.</p>
  
  <p>workbook is initiate with below:</p>
  
  <pre><code>workbook = xlsxwriter.Workbook(r'\\network\Checking doc\Monthly Checks.xlsx', {'constant_memory' : False})
  </code></pre>
  
  <p>Cell formatting is handled by:</p>
  
  <pre><code>merge_header = workbook.add_format({'align' : 'center', 'valign' : 'vcenter', 'border' : 1})
  merge_header.set_bg_color('#BFBFBF')
  </code></pre>
  
  <p>The merging of cells uses a for loop in a for loop which looks through 2 lists to get the cell coordinates.</p>
  
  <p>My lists don't have good names I know but they are:</p>
  
  <pre><code>other_checkheads = ["C6 - LOGS", "C7 - HWS", "C8 - TMP", "C9 - WUP", 
  
  "C10 - SWUP", "C11 - ADGP", "C12 - FWSWFIRM", "C13 - WBFIL", "C14 - FILES", "C15 - DOMPASS", "C16 - NAT",
  "C17 - SECGP", "C18 - BIOS", "C19 - BIOSFW", "C20 - MOT", "C21 - SSL", "C22 - WARR"]
  checkheads_letters = ["Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z", "AA", "AB", "AC", "AD", "AE", "AF", "AG"]
  </code></pre>
  
  <p>The loop is:</p>
  
  <pre><code>for letter in checkheads_letters:
                  for head in other_checkheads:
                      row = ((letter + str(2)) + (":") + (letter +str(4)))
                      worksheet.merge_range(row, head, merge_header)
                      print (row)
  </code></pre>
  
  <p>When I run this and try open the document I receive the error:
  "We found a problem with some content in 'Monthly Checks.xlsx'. Do you want us to try recover as much as we can? If you trust the source of this workbook, click Yes.</p>
  
  <p>I click yes and then get a file in use error stating the document is locked for editing by 'another user'. If I select to notify the person it then opens the document successfully with a note on the repairs which were made.</p>
  
  <p>If I completely remove the fr loops the document opens without any errors.</p>