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In addition to keeping track of types, the type machinery also locates the global variables (roots) that the garbage collector starts at. Roots must be declared using one of the following syntaxes:
extern GTY(([options])) type name;
static GTY(([options])) type name;
GTY(([options])) type name;
extern declaration
of such a variable in a header somewhere—mark that, not the
definition. Or, if the variable is only used in one file, make it
static.