Problem solving most simple elliptic PDE, 2015a

Trying to solve $\Delta u=1$ on the unit square, I currently do

 pdem = createpde(1);
 c=1;
 a=0;
 f=1;
 geometryFromEdges(pdem,@squareg);
 generateMesh(pdem,'Hmax',0.2);
 assempde(pdem,c,a,f);

Unfortunately, the last command does not work, since it requires additionally the specification of variables P, E,T, which according to the help output specify a mesh.

Why do I need to specify the mesh again in order to solve the PDE? HOW do I specify the mesh? 'generateMesh' only gives one output.