function?[seisbc_topbc_bottombc_leftbc_rightbc_p_ntbc_p_nt_1]...
????=a2d_mod_abc24（vnbcdxntdtssxszgxgzisFS）
%??Copyright?（C）?2010?Center?for?Subsurface?Imaging?and?Fluid?Modeling?（CSIM）
%??King?Abdullah?University?of?Science?and?Technology?All?rights?reserved.
%
%??author:???Xin?Wang
%??email:????xin.wang@kaust.edu.sa
%??date:?????Sep?26?2012
%??purpose:??2DTDFD?solution?to?acoustic?wave?equation?with?accuracy?of?2-8
%????????????use?the?absorbing?boundary?condition
%
%??IN???v（::）?--?velocity??????nbc?????????--?grid?number?of?boundary
%???????dx?????--?grid?intervel?nt??????????--?number?of?sample
%???????dt?????--?time?interval?s（:）????????--?wavelet
%???????sxsz??--?src?position??gx（:）gz（:）?--?rec?position
%???????isFS???--?if?is?Free?Surface?condition
%??OUT??seis（::）??????????--?Output?seismogram
%???????bc_top（:::）??????--?array?for?storing?top?boundary?condition
%???????bc_bottom（:::）???--?array?for?storing?bottom?boundary?condition
%???????bc_left（:::）?????--?array?for?storing?left?boundary?condition
%???????bc_right（:::）????--?array?for?storing?right?boundary?condition
%???????bc_p_nt（::）???????--?array?for?storing?the?last?time?step?wavefield
%???????bc_p_nt_1（::）?????--?array?for?storing?the?second?last?time?step?wavefield

seis=zeros（ntnumel（gx））;[nznx]=size（v）;
ng=numel（gx）;
c1?=?-2.5;?c2?=?4.0/3.0