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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def format_sympy_expr(sympy_expr, functions=None):
 """Convert sympy expression into a string which can be encoded. Args: sympy_expr: Any sympy expression tree or string. functions: Defines special functions. A dict mapping human readable string names, like "log", "exp", "sin", "cos", etc., to single chars. Each function gets a unique token, like "L" for "log". Returns: A string representation of the expression suitable for encoding as a sequence input. """

  if functions is None:
    functions = {}
  str_expr = str(sympy_expr)
  result = str_expr.replace(" ", "")
  for fn_name, char in six.iteritems(functions):
    result = result.replace(fn_name, char)
  return result