Procedure from library linalg.lib (see linalg_lib).
linalg.lib
jordanmatrix(e,s,m); ideal e, intvec s, intvec m
ncols(e)==size(s)==size(m)
matrix J; Jordan matrix with list(e,s,m)==jordan(J)
Example:
LIB "linalg.lib"; ring R=0,x,dp; ideal e=ideal(2,3); intvec s=1,2; intvec m=1,1; print(jordanmatrix(e,s,m));