A FlagMatroid with constituent matroids $\{M_1, \ldots, M_k\}$ is well-defined if $M_i$ is a matroid quotient of $M_{i+1}$ (i.e. every flat of $M_i$ is a flat of $M_{i+1}$) for all $i = 1, \ldots, k-1$.
i1 : FM = flagMatroid {uniformMatroid(2,4),uniformMatroid(3,4)}
o1 = a "flag matroid" with rank sequence {2, 3} on 4 elements
o1 : FlagMatroid
i2 : isWellDefined FM
o2 = true
i3 : FMbad = flagMatroid {uniformMatroid(2,4), uniformMatroid(1,2)++uniformMatroid(2,2)}
o3 = a "flag matroid" with rank sequence {2, 3} on 4 elements
o3 : FlagMatroid