DIFFERENCE OF OPINION AS TO CLRNC TO CROSS RWY AT LAX.

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I WOULD LIKE TO INFORM YOU OF THE FACTS THAT OCCURRED AT LAX ARPT. AFTER WE LANDED ON RWY 25L AT LAX; THE TWR CTLR INSTRUCTED US DURING THE LNDG TO CLR THE RWY 25L VIA TXWY N AND CROSS RWY 25R AND REMAIN ON TWR FREQ. THESE INSTRUCTIONS WERE READ BACK BY THE FO WITHOUT RECEIVING FURTHER COMS FROM THE TWR CTLR. WHILE BEING AT TXWY N AND AFTER THE RWY 25R WAS CLRED; WE STARTED XING ACCORDING TO THE INSTRUCTIONS AND AT THE MIDDLE OF THE RWY 25R; THE TWR CTLR THEN GAVE US THE INSTRUCTION TO HOLD SHORT OF RWY 25R. WE COULDN'T COMPLY WITH THE INSTRUCTION BECAUSE THE AIRPLANE WAS IN THE MIDDLE OF THE RWY; AS WE MENTIONED BEFORE. THAT'S WHY WE EXPEDITED XING THE RWY. AT THAT MOMENT WE SAW ABOVE US AN ACFT; APPARENTLY A B757. LATER THE TWR CTLR SAID THAT THE INSTRUCTION WAS TO HOLD SHORT OF THE RWY AND REMAIN ON FREQ. THE FO TOLD THEM THAT HE READ BACK THE INSTRUCTIONS TO CLR THE RWY 25L VIA TXWY N AND CROSS RWY 25R REMAINING ON TWR CTL FREQ. AFTER THAT; THE TWR CTLR INSTRUCTED US TO CHANGE THE FREQ AND THEY GAVE US A PHONE NUMBER TO CONTACT THEM.