fopen.sheet

  r   Read-only, starts at beginning of file (default mode).
  r+  Read-write, starts at beginning of file.
  w	  Write-only, truncates existing file to zero length or creates a new file for writing.
  w+  Read-write, truncates existing file to zero length or creates a new file for reading and writing.
  a	  Write-only, starts at end of file if file exists, otherwise creates a new file for writing.
  a+  Read-write, starts at end of file if file exists, otherwise creates a new file for reading and writing.
  b	  (DOS/Windows only) Binary file mode (may appear with any of the key letters listed above).