B737-700 FLT CREW HAS A TCAS RA DURING SADDE SIX ARR TO LAX.

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ON SADDE SIX ARR TO LAX CLRED TO CROSS SYMON AT 12000 FT. NOTIFIED BY ZLA OF VFR TFC AT 11-12 O'CLOCK POS; AT 14000 FT MSL. MAINTAINED NORMAL DSCNT GRADIENT TO CROSS SYMON AT 12000 FT. RECEIVED TCAS TA; AND SHORTLY THEREAFTER; AN RA TO MAINTAIN VERT SPD (APPROX 2000 FPM DSCNT). I ACQUIRED VISUAL CONTACT OF THE TFC AT 12 O'CLOCK POS; AND AS WE DSNDED THROUGH THE TFC'S ALT AND WERE BELOW THE OTHER ACFT. IT APPEARED TO ME AS IF HE BEGAN A DSCNT AND WE RECEIVED A TCAS RA TO CLB. SINCE THE RATE OF CLOSURE WAS HIGH AND WE WERE ALREADY BELOW THAT ACFT AND DSNDING; I MAINTAINED THE DSCNT TO AVOID PULLING BACK UP THROUGH THE CONFLICTING TFC'S ALT AND AVOID AN NMAC. WE WERE ABLE TO MAINTAIN THE ATC CLRNC AND COMPLY WITH THE RA GUIDANCE EXCEPT WITH RESPECT TO REVERSING THE DSCNT WHEN BELOW AND CLR OF THE OTHER TFC. ATC COULD HAVE DIRECTED US TO LEVEL OFF ABOVE THE TFC AND THEN DELETE THE SYMON XING RESTR; I COULD ALSO HAVE EITHER INCREASED MY RATE OF DSCNT TO GET BELOW THE TFC OR LEVELED OFF ABOVE AND REQUESTED RELIEF FOR THE SYMON XING RESTR.