The header conforms to the DIMACS standard which is as follows:
p bdd <#vars> <#functions>
The letter 'p' must appear at the start of the
header. The second header token, which in this case is 'bdd',
identifies the input format. The header contains two integer fields:
<#vars> is the number of distinct variables contained in the
file; <#functions> is the number of Boolean functions (including
manipulators) contained in the file. The following is an example of a
valid header for a file containing 97 functions composed from 56
distinct variables.
p bdd 56 97