permuteinstrata: Randomize the order of elements *within* each stratum 

ppss: Use PPS systematic sampling to select a sample of n units out of N 

ppssstrat(stratified pps systematic sampling): Apply ppss() to each stratum

ppswr: Select a PPS sample *with* replacement of n units out of N

sampford: Sampford's method for PPS sampling

sampfordpi: Compute joint inclusion probabilities for Sampford's method

stratsrs: Select a simple random sample from each stratum
