  We focus on the strategyproofness of voting systems where voters must choose
a number of options among several possibilities. These systems include those
that are used for Participatory Budgeting, where we organize an election to
determine the allocation of a community's budget (city, region, etc.) dedicated
to the financing of projects.
  We present a model for studying voting mechanisms and the Constrained Change
Property (CCP), which will be used to design voting mechanisms that are always
strategyproof. We also define a new notion of social choice function and use it
to design a new class of utilitarian voting mechanisms that we call score
voting. We prove that the mechanisms designed with core voting with a neutral
score function are equivalent to knapsack voting on the same instance and that
any score voting designed with a total score function is strategyproof if and
only if its score function satisfies CCP.
  These results are combined to devise an algorithm that can find the closest
total score function that makes any given score voting to be strategyproof.
