! Gauss-Seidel iteration for Poisson's equation
! =============================================

USE gnuplot   ! prepare for plotting
ARRAY(0..20,0..40) OF REAL var,rhs   ! declare and allocate ARRAY variables

var=0; rhs=0
DO var(i,HI)=1 FOR ALL i   ! assign some boundary conditions

LOOP FOR 100 TIMES   ! main iteration loop
  LOOP FOR i=LO+1 TO HI-1 AND j=LO+1 TO HI-1
    var(i,j)=[var(i,j+1)+var(i,j-1)+var(i+1,j)+var(i-1,j) + rhs(i,j)]/4
  REPEAT LOOP   ! these three lines are the core iteration
REPEAT LOOP

SPLOT var
READ ! keep plot displayed until Enter is pressed

!(
  A single LOOP instruction can loop over several indices. Indices are
  implicitly declared. Various LOOP and DO forms are available.
  Round, square and curly brackets can be used like in mathematics.
!)