exit status: 1 with stderr:
<stdin>:4:16: error: immutable value 'self.a' may only be initialized once self.a = a ^ <stdin>:2:9: note: initial value already provided in 'let' declaration let a = 5 ^ <stdin>:2:5: note: change 'let' to 'var' to make it mutable let a = 5 ^~~ var