from mpictor's commit fixing a crash from newer versions of flex, mpictor/StepClassLibrary@37ac5dc; which is from a gentoo spim patch, which seems to have originated from a fedora patch. apparently yy_init was changed from meaning 'needs to be initialized' to 'was initialized', effectively flipping the boolean meaning.

Christopher Sean Morrison · Christopher Sean Morrison · commit 8a9c6a791838 · 2011-07-10T19:16:06.000Z
diff --git a/src/express/expscan.l b/src/express/expscan.l
@@ -434,6 +434,11 @@ exp_flex_init()
 {
 	/* yy_init is what tells flex to reinitialize its buffers */
 
+#  if (YY_FLEX_MAJOR_VERSION==2 && YY_FLEX_MINOR_VERSION==5 && YY_FLEX_SUBMINOR_VERSION>=33)
+	/* flex 2.5.33 flips the polarity of this flag */
+	yy_init = 0;
+#  else
 	yy_init = 1;
+#  endif
 }
 #endif

Original file line number	Diff line number	Diff line change
`@@ -434,6 +434,11 @@ exp_flex_init()`
`434`	`434`	`{`
`435`	`435`	`/* yy_init is what tells flex to reinitialize its buffers */`
`436`	`436`
	`437`	`+# if (YY_FLEX_MAJOR_VERSION==2 && YY_FLEX_MINOR_VERSION==5 && YY_FLEX_SUBMINOR_VERSION>=33)`
	`438`	`+ /* flex 2.5.33 flips the polarity of this flag */`
	`439`	`+ yy_init = 0;`
	`440`	`+# else`
`437`	`441`	`yy_init = 1;`
	`442`	`+# endif`
`438`	`443`	`}`
`439`	`444`	`#endif`