RWY INCURSION AS BE40 FLC CROSSES THE HOLD SHORT LINE BEFORE STOPPING AT SDM; CA.

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WE WERE TAXIING TO RWY 26R AT SDM AND TWR ASKED US IF WE WOULD BE READY FOR DEP AT THE END. I REPLIED WE WOULD NOT BE. TWR THEN INSTRUCTED US TO TAXI ACROSS A PARALLEL TXWY BUT TO HOLD SHORT OF RWY 26R. THIS WOULD PREVENT US FROM BLOCKING THE TXWY AND ALLOW ANOTHER ACFT TO DEPART BEFORE US. I READ THIS BACK AND THEN WENT BACK TO SETTING UP THE NAV EQUIP. THE CAPT THEN BECAME CONFUSED AND WRONGLY UNDERSTOOD OUR CLRNC AS A CLRNC TO TAXI ACROSS THE RWY IN ORDER TO ALLOW FOR THE FOLLOWING ACFT TO DEPART BEFORE US. WHILE I CALLED TWR TO VERIFY OUR TAXI INSTRUCTIONS; THE CAPT CONTINUED TO TAXI AND STOPPED SLIGHTLY PAST THE HOLD SHORT LINE FOR RWY 26R. I POINTED THIS OUT TO HIM AND HE REALIZED HIS MISTAKE. THE TIP OF THE NOSE WAS 2-3 FT PAST THE HOLD SHORT LINE. BECAUSE IT WAS SUCH A SLIGHT BREACH OF THE HOLD SHORT LINE (THE TIP OF OUR NOSE); IT WENT UNNOTICED AS FAR AS I KNOW. THE CTL TWR DIDN'T SAY ANYTHING. THE CAPT SHOULD HAVE STOPPED AND NOT CONTINUED TO TAXI UNTIL I WAS THROUGH VERIFYING OUR TAXI CLRNC WITH ATC. IT WAS VERY DARK AND THEREFORE WE SHOULD'VE BEEN VIGILANT.