std.range.equal or == in isPalindrome

[Per Nordlöw" <per.nordlow at gmail.com>]

I'm curious to why we need std.range.equal in cases such as

bool isPalindrome(Range)(in Range range) if 
(isBidirectionalRange!Range)
{
     return range.retro.equal(range);
}

Why isn't equality == operator used here instead?

/Per