disable_function_contracts()          
      

function f(x) where is_number(x) -> is_string(%)
  string_append("hello ", number_to_string(x))

f(10)
// hello 10
f('abc)
//> error: precondition_failed, f, (is_number x)

disable_function_contracts()
f('abc)
//> error: (Argument 1) NUMBER expected
//> (number->string 'abc)