digraph categories {
    graph [nodesep=.75];

    expression [shape=plaintext, height=.1, fontsize=24];
    glvalue [shape=plaintext, height=.1, fontsize=24];
    lvalue [shape=plaintext, height=.1, fontsize=24];
    xvalue [shape=plaintext, height=.1, fontsize=24];
    rvalue [shape=plaintext, height=.1, fontsize=24];
    prvalue [shape=plaintext, height=.1, fontsize=24];

    expression -> glvalue;
    glvalue -> lvalue;
    glvalue -> xvalue;
    expression -> rvalue;
    rvalue -> xvalue;
    rvalue -> prvalue;
}