P1+/P1 bubble function in FEM
 

I am looking for hints to implement a P1+/P1 MINI element in a FEM code; the idea is to use equal order interpolation for velocity and pressure (linear triangles in 2D, linear tetrahedrons in 3D).

Does anyone have a good reference with explicit hints concerning the condensation of the bubble variable and the implementation into the code ?

Any ideas are welcome ...