I tried the following do loop

 for k from 1 to 4 do

s:=0

xmp:=lstt[indlax[k]]

s:=ym(x)+s

end do

where xmp is getting a numeric value from list "lstt" for each k

ym(x) is a piecewise function containing unknowns x and xmp,

what I would like to get is ym(x) for each k, and sum them up