! 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

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.