Is self a letter? (from 'A' to 'Z' and 'a' to 'z')

assert 'A'.is_letter  == true
assert 'A'.is_letter  == true
assert 'z'.is_letter  == true
assert '$'.is_letter  == false

Property definitions

core $ Char :: is_letter
	# Is self a letter? (from 'A' to 'Z' and 'a' to 'z')
	#
	#     assert 'A'.is_letter  == true
	#     assert 'A'.is_letter  == true
	#     assert 'z'.is_letter  == true
	#     assert '$'.is_letter  == false
	fun is_letter : Bool
	do
		return is_lower or is_upper
	end
lib/core/kernel.nit:1040,2--1049,4