###
#   r   Open a file for reading. The file must exist.
#
#   w   Create an empty file for writing. If a file with the same name 
#       already exists its content is erased and the file is treated as a new 
#       empty file.
#
#   a   Append to a file. Writing operations append data at the end of the 
#       file. The file is created if it does not exist.
#
#   r+  Open a file for update both reading and writing. The file must exist.
#
#   w+  Create an empty file for both reading and writing. If a file with the 
#       same name already exists its content is erased and the file is 
#       treated as a new empty file.
#
#   a+  Open a file for reading and appending. All writing operations are 
#       performed at the end of the file, protecting the previous content 
#       from being overwritten. 
#
#   You can reposition (fseek, rewind) the internal pointer to anywhere in the 
#   file for reading, but writing operations will move it back to the end of 
#   file. The file is created if it does not exist.
###
File.open(local_filename, 'w') {|f| f.write(doc) }