Procs

proc expm1(x: float32): float32 {.importc: "expm1f", header: "<math.h>",
                                  ...raises: [], tags: [], forbids: [].}

  Source Edit

proc expm1(x: float64): float64 {.importc: "expm1", header: "<math.h>",
                                  ...raises: [], tags: [], forbids: [].}

Compute exp(x) - 1 and avoids catastrophic cancellation if x ~= 0 i.e. if x ~= 0 exp(x) - 1 ~= x but normal float rounding would do exp(0) - 1 = 0 instead.   Source Edit

proc ln1p(x: float32): float32 {.importc: "log1pf", header: "<math.h>",
                                 ...raises: [], tags: [], forbids: [].}

  Source Edit

proc ln1p(x: float64): float64 {.importc: "log1p", header: "<math.h>",
                                 ...raises: [], tags: [], forbids: [].}

Compute ln( 1+x ) and avoids catastrophic cancellation if x << 1 i.e. if x << 1 ln(1+x) ~= x but normal float rounding would do ln(1) = 0 instead.   Source Edit