```ruby
module TChart
  module CommandLineParser
    
    def self.parse(argv) # => CommandLineArgs, errors
      data_filename, tex_filename = argv
      errors = validate_args(argv, data_filename, tex_filename)
      [ CommandLineArgs.new(data_filename, tex_filename), errors ]
    end
    
  private
  
    def self.validate_args(argv, data_filename, tex_filename)
      errors = []
      if argv.length != 2
        errors << "Usage: tchart input-data-filename output-tikz-filename"
      elsif ! File.exists?(data_filename)
        errors << "Error: input data file \"#{data_filename}\" not found."
      elsif ! File.file?(data_filename)
        errors << "Error: input data file \"#{data_filename}\" is not a file."
      elsif File.exists?(tex_filename) && ! File.file?(tex_filename)
        errors << "Error: existing output data file \"#{tex_filename}\" is not a file."
      elsif same_file?(data_filename, tex_filename)
        errors << "Error: input \"#{data_filename}\" and output \"#{tex_filename}\" refer to the same file."
      end
      errors
    end

    def self.same_file?(filename1, filename2)
      File.expand_path(filename1) == File.expand_path(filename2)
    end

  end
end
```

Add comment about method return value.
user
```ruby
module TChart
  module CommandLineParser
    
    def self.parse(argv) # => CommandLineArgs, errors
      data_filename, tex_filename = argv
      errors = validate_args(argv, data_filename, tex_filename)
      [ CommandLineArgs.new(data_filename, tex_filename), errors ]
    end
    
  private
  
    def self.validate_args(argv, data_filename, tex_filename) # => errors
      errors = []
      if argv.length != 2
        errors << "Usage: tchart input-data-filename output-tikz-filename"
      elsif ! File.exists?(data_filename)
        errors << "Error: input data file \"#{data_filename}\" not found."
      elsif ! File.file?(data_filename)
        errors << "Error: input data file \"#{data_filename}\" is not a file."
      elsif File.exists?(tex_filename) && ! File.file?(tex_filename)
        errors << "Error: existing output data file \"#{tex_filename}\" is not a file."
      elsif same_file?(data_filename, tex_filename)
        errors << "Error: input \"#{data_filename}\" and output \"#{tex_filename}\" refer to the same file."
      end
      errors
    end

    def self.same_file?(filename1, filename2)
      File.expand_path(filename1) == File.expand_path(filename2)
    end

  end
end
```
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