Before revision:  We exhibit a general model for absolute discrimination , and further show how addition of proofreading steps inverts the expected hierarchy of antagonism without fully cancelling it. Phenotypic spandrels reveal the internal feedbacks and constraints structuring response in signalling pathways, in very similar way to symmetries structuring physical laws\epsilonRIs .
After revision:  We exhibit a general model for absolute discrimination , and further show how addition of proofreading steps inverts the expected hierarchy of antagonism compared to simple model . The phenotypic spandrel studied here is expected to structure many decision pathways such as immune detection mediated by TCRs and Fc\epsilonRIs .