RWY INCURSION AFTER A TXWY INCURSION TO TXWY B VERSUS TXWY P BY AN A319 FLT CREW WHO WAS DISTR WITH A XBLEED START ON TAXI OUT TO RWY 13 AT LGA.

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AS WE TAXIED TO THE RWY AT LGA GND CTL TOLD US TO TAXI TO RWY 13 VIA BEHIND ACR Y TXWY B TO TXWY P TO HOLD SHORT OF RWY 22 AT TXWY P. AS WE TAXIED WE COMPLETED A XBLEED START AND I WAS BUSY AS WE APCHED TXWY P WHICH THE CAPT MISSED. BEFORE I COULD CONTACT GND TO SEE WHAT HE WANTED US TO DO HE TOLD US TO TURN ONTO TXWY F. FROM THE POS FROM WHICH THE CAPT MADE THE TURN THE CAPT COULD NOT SEE THE HOLD SHORT LINES WHICH WE CROSSED SLIGHTLY. GND TOLD US TO STOP WHICH WE DID. THERE MAY HAVE BEEN A GAR AS A RESULT. GND DID NOT SAY THERE WAS ANY PROB OR CONFLICT. SUPPLEMENTAL INFO FROM ACN 626122: AFTER A FEW MAINT PROBS AT THE GATE (APU AIRBLEED; APPROX 100% IN COCKPIT EXTERNAL AIRSTART; XBLEED START; FLT ATTENDANT PANEL; SECURITY INSPECTION) WE WERE TOLD TO FOLLOW ACFT Y ACR B767. I EITHER DID NOT HEAR 'HOLD SHORT OF RWY 22 AT TXWY P;' OR I FORGOT THE INSTRUCTIONS. AS I MADE THE TURN; LOOKING FOR THE HOLD SHORT LINES; THE FO SAID WE HAD ALREADY PASSED THEM. ATC DID NOT QUESTION US AS TO OUR UNDERSTANDING OF THE TAXI INSTRUCTIONS.