r - How can I specify 'unstructured' in lme function? -

r - How can I specify 'unstructured' in lme function? -

i'm trying fit mixed model in r. have stata code , want same thing using r. stata code is

xtmixed laz c.x1##i.j4alloc_n c.x2##i.j4alloc_n ||childuid:age_m, cov(uns) var

i can't find how specify 'unstructured' correlation construction in r. in r help file, there no 'unstructured' https://stat.ethz.ch/r-manual/r-devel/library/nlme/html/corclasses.html

r nlme