input: Write a function that takes a list of strings and returns the concatenation of all the elements with a specified separator. If no separator is given, a default of a comma should be used.
  
  def concatenate_strings(strings, separator=',')
    return separator.join(strings)
  
  assert concatenate_strings(["Sample","Text","Hello"], "/")=="Sample/Text/Hello"
  assert concatenate_strings(["Sample","Text","Hello"])=="Sample,Text,Hello"
  assert concatenate_strings([], ";")==""
  assert concatenate_strings(["Single"])=="Single"
  assert concatenate_strings(["First", "Second", "Third"], "-")=="First-Second-Third"
output: def concatenate_strings(strings, separator=','):
    return separator.join(strings)