In graph theory, an interval I(h) in a directed graph is a maximal, single entry subgraph in which h is the only entry to I(h) and all closed paths in I(h) contain h. Intervals were described in 1976 by F. E. Allen and J. Cooke. Interval graphs are integral to some algorithms used in compilers, specifically data flow analyses. The following algorithm finds all the intervals in a graph consisting of vertices N and the entry vertex n0, and with the functions pred(n) and succ(n) which return the list of predecessors and successors of a given node n, respectively.