This disclosure relates to the analysis of a program based on source code where the source code comprises a call to a function associated with a function implementation. A processor determines, based on a summary that over-approximates the function, an assignment of an input variable and an output variable of the function call to reach a predefined state. The processor then determines, based on the implementation of the function whether the assignment of the input variable results in the assignment of the output variable. If it does not, the processor determines a narrowed summary for the function such that the narrowed summary over-approximates the function and excludes the assignment of the input variable and the output variable. Finally, the processor stores the narrowed summary on a datastore. Inlining of function code and unfolding of loops is avoided and parallel processing of multiple functions is possible.