code: def format_sympy_expr(sympy_expr, functions=None):
    
    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
docs: 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.