<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def simple_cnn(actns:Collection[int], kernel_szs:Collection[int]=None,
               strides:Collection[int]=None, bn=False) -> nn.Sequential:
    "CNN with `conv_layer` defined by `actns`, `kernel_szs` and `strides`, plus batchnorm if `bn`."
    nl = len(actns)-1
    kernel_szs = ifnone(kernel_szs, [3]*nl)
    strides    = ifnone(strides   , [2]*nl)
    layers = [conv_layer(actns[i], actns[i+1], kernel_szs[i], stride=strides[i],
              norm_type=(NormType.Batch if bn and i<(len(strides)-1) else None)) for i in range_of(strides)]
    layers.append(PoolFlatten())
    return nn.Sequential(*layers)