Create a function that takes a string and determine if it's a valid sequence by either returning True or False. The string will be composed of + and = symbols with several characters between them (e.g. "++d+===+c++==a") and for the string to be True, each letter must be surrounded by a + symbol. So the string to the left would be False.
simple_symbols("f++d+") ➞ False
simple_symbols("+d+=3=+s+") ➞ True
simple_symbols("==+p+++++++++====8+z++++") ➞ True
The given string will not be empty and will have at least one letter.