To add an arrow to an axe

edit

Here is my attempt to add an arrow head to the axes of a graph. It seems not very effective since when I change the localization of the arrow the axes change also

far = plot(x^0.4,(x, 0, 10),ticks=[[], []])
far+= plot(1-exp(-0.4*x),(x, 0, 10),color="red",ticks=[[], []],ymin=0,ymax=3)
far+=text("$U$",(-.5,3),color="black", fontsize='small')
far+=text("$x$",(10.5,-.05),color="black", fontsize='small')
far+=text("$U=x^a$",(9.6,2.6),color="blue", fontsize='small', rotation=18)
far+=text("$U=1-e^{-0.4*x}$",(9.6,1.1),color="red", fontsize='small', rotation=5)
far+=arrow((10.4,0),(10.5,0),color='black',arrowsize=1, thickness =.5 )
far

In many ploting software arrows are option to objects. It seems that it is not the case in SM