AMPL defined variables

AMPL has a construct called defined variables that can sometimes reduce the size of an NLP by a large amount. Here are some statistics for a client model (confidential so I cannot share it):

GAMS:

BLOCKS OF EQUATIONS           6     SINGLE EQUATIONS       26,192
BLOCKS OF VARIABLES           5     SINGLE VARIABLES       26,201
NON ZERO ELEMENTS       119,031     NON LINEAR N-Z         45,220
DERIVATIVE POOL               8     CONSTANT POOL           1,541
CODE LENGTH             359,381

AMPL:

Substitution eliminates 26180 variables.
Adjusted problem:
10 variables, all nonlinear
1 constraint, all linear; 10 nonzeros 
1 nonlinear objective; 10 nonzeros.

GAMS has some similar facilities built in the LGO solver, but these are not available as a general feature.