reg is either [rx,ry] or ind(ix,iy)
REG(OP,reg) := if (reg=[rx,ry]) x,y=rx,ry
if (reg=ind(ix,iy)) val_reg(ix,iy) get_2d(reg[ix,iy],x,y)
val_reg(x,y) OP(x,y)
STO(reg) := REG(sto, reg)
RCL(reg) := REG(rcl, reg)
Exc(reg) := REG(exc, reg)
SUM(reg) := REG(sum, reg)
iSUM(reg) := REG(isum, reg)
Prd(reg) := REG(prd, reg)
iPrd(reg) := REG(iprd, reg)
sto(x,y) := reg[x,y] = X
rcl(x,y) := X = reg[x,y]
exc(x,y) := X <=> reg[x,y]
sum(x,y) := reg[x,y] = plus(reg[x,y], X)
isum(x,y) := reg[x,y] = minus(reg[x,y], X)
prd(x,y) := reg[x,y] = times(reg[x,y], X)
iprd(x,y) := reg[x,y] = divides(reg[x,y], X)
