Question about robustreg

[CBL]

I am trying to figure out how to test parameters while using robust regressions (proc robustreg). That is, I would like to compare the values of several parameters with each other.

For a standard OLS model, one can write

test X1-X2=X3-X4; OR
text X1-X2-X3+X4;

Neither works for me. Any suggestions?