What is the >>>= operator in C?

Given by a colleague as a puzzle, I cannot figure out how this C program actually compiles and runs. What is this >>>= operator and the strange 1P1 literal? I have tested in Clang and GCC. There are no warnings and the output is “???”

#include <stdio.h>

int main()
{
    int a[2]={ 10, 1 };

    while( a[ 0xFULL?'\0':-1:>>>=a<:!!0X.1P1 ] )
        printf("?");

    return 0;
}