SortedRange.lowerBound from FrontTransversal

[Alex via Digitalmars-d-learn]

Hi all... a technical question from my side...
why the last line of the following gives the error?

import std.stdio;
import std.range;
import std.algorithm;

void main()
{
     size_t[][] darr;
     darr.length = 2;
     darr[0] = [0, 1, 2, 3];
     darr[1] = [4, 5, 6];
     auto fT = frontTransversal(darr);
     assert(equal(fT, [ 0, 4 ][]));

     auto heads = assumeSorted!"a <= b"(fT);
     writeln(heads.lowerBound(3)); //!(SearchPolicy.gallop)
}

The error is:
Error: template 
std.range.SortedRange!(FrontTransversal!(ulong[][], 
cast(TransverseOptions)0), "a <= b").SortedRange.lowerBound 
cannot deduce function from argument types !()(int), candidates 
are:
package.d(7807,10): 
std.range.SortedRange!(FrontTransversal!(ulong[][], 
cast(TransverseOptions)0), "a <= 
b").SortedRange.lowerBound(SearchPolicy sp = 
SearchPolicy.binarySearch, V)(V value) if 
(isTwoWayCompatible!(predFun, ElementType!Range, V) && 
hasSlicing!Range)

I tried also with "assumeNotJagged" for the FrontTransversal, it 
didn't worked either, beyond the fact, that assumeNotJagged is 
not of interest for me...