It's a Command so it may be entered simply as engineStack if it's alone on the command line.
Used mainly for debugging the engine.
i1 : ZZ/101[x,y,z] ZZ o1 = ---[x,y,z] 101 o1 : PolynomialRing
i2 : f = matrix {{x,y,z}} o2 = {0} | x y z | ZZ 1 ZZ 3 o2 : Matrix (---[x,y,z]) <--- (---[x,y,z]) 101 101
i3 : sendgg ggPush f o3 = o3 : String
i4 : engineStack stack: Matrix x y z