// A not passed pawn is a candidate to become passed if it is free to
// advance and if the number of friendly pawns beside or behind this
// pawn on adjacent files is higher or equal than the number of
// enemy pawns in the forward direction on the adjacent files.
candidate = !(opposed | passed | backward | isolated)
// A not passed pawn is a candidate to become passed if it is free to
// advance and if the number of friendly pawns beside or behind this
// pawn on adjacent files is higher or equal than the number of
// enemy pawns in the forward direction on the adjacent files.
candidate = !(opposed | passed | backward | isolated)