Default variables not supported in filtering

[jeromekelleher]

bcftools supports these variables in expressions, computed on the fly if necessary https://samtools.github.io/bcftools/bcftools.html

variables calculated on the fly if not present: number of alternate alleles; number of samples; 
count of alternate alleles; minor allele count (similar to AC but always picks the allele with
 frequency smaller than 0.5); frequency of alternate alleles (AF=AC/AN); frequency of 
minor alleles (MAF=MAC/AN); number of alleles in called genotypes; number of samples with 
missing genotype; fraction of samples with missing genotype; indel length (deletions negative, 
insertions positive, balanced substitutions zero)

N_ALT, N_SAMPLES, AC, MAC, AF, MAF, AN, N_MISSING, F_MISSING, ILEN