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As I somehow forgot but Szabolcs reminded me many System variables (typically with names beginning with $) check the values they are set to, e.g.:$MinPrecision := -6 $MinPrecision \$MinPrecision::precset: Cannot set \$MinPrecision to -6; value must be a non-negative number or Infinity. >> 0 In this case the check causes evaluation you do not ... 4 To change the assumptions dynamically, this can be used:$Assumptions := b b = {c > 0}; Refine[{c < 0, c == 0, c > 0}] b = {c < 0}; Refine[{c < 0, c == 0, c > 0}] If you need to evaluate the Print every time, this can be used: \$Assumptions := Evaluate[b] b = {c > 0, Unevaluated@Print[kount]}; kount = 1; Refine[{c < 0, c == 0, c > 0}] kount = 2; ...