Hi, I have a program which needs to recalculate a formula of independant arguments (doubles or integers). These arguments can be updated at any time. This math formula is expensive to evaluate, and performance-wise it is worth, depending of which input argument changes, to recalculate only a part of the evaluation tree. Here is a simplistic exemple: z = sin(x+1)*cos(y-3) When the argument x changes, it is not necessary to recompute cos(y-3). I wrote a prototype lib to do this, where expressions are made of cells wrapping operators, input cells and intermediate values, plus the list of depending cells. When the value of an argument, an input cell, changes, the lib just builds the topologically sorted list of dependant cells, and reevaluates them, performing what I think is the minimum recalculation (Excel style). (An alternate strategy is to let each data update invalidate its cell and its descendants - the actual calculation being done only when needed). Instead of reinventing the wheel, my thought was to try to use boost::proto for this. Does it make sense ? Any code change needed please ? Thanks