const 
    alphanumerics = "0123456789_abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
proc alpha(n: char) bool = # Invalid indentation here.
    if n in alphanumerics:
        return true
    else:
        return false

echo alpha(n)