'begin' # 8.14. Runge-Kutta method #
    'proc'RK=('real'x0,y0,'proc'('real','real')'real'f,'real'x1,eps)'real':
       ('proc'RK1=('real'x,y,h)'real':
	   ('real'z:=h*f(x,y);'real'res:=z;
	    z:=h*f(x+h/2,y+z/2);res+:=2*z;
	    z:=h*f(x+h/2,y+z/2);res+:=2*z;
	    y+(res+h*f(x+h,y+z))/6
	   );
	'real'h:=(x1-x0)/2,x:=x0,y:=y0;'real'y1,y2,y3;
	'bool'out:='true';'int'c:=0;
L1:	y1:=RK1(x,y,2*h);
L2:	y2:=RK1(x,y,h);y3:=RK1(x+h,y2,h);
	'if''abs'(y3-y1)>eps'then'y1:=y2;h/:=2;c:=0;out:='false';L2'fi';
	'if'out'then'y3
	'else'
	    x+:=2*h;y:=y3;'if'(c+:=1)>5'then'c:=0;h*:=2'fi';
	    'if''abs'(x1-x)<2.01*'abs'h'then'
		out:='true';h:=(x1-x)/2
	    'fi';
	    L1
	'fi'
       );
    print(RK(0,1,('real'x,y)'real':y,3,.00001));
    print((exp(3),newline));
    print(RK(0,0,('real'x,y)'real':x+y,3,.00001));
    print((exp(3)-4,newline))
'end'