G2P: Added fallback rules to prevent illegal results

src/recognition/g2pRules.cpp

@@ -206,3 +206,10 @@
 { wregex(L"l(l)"), L"$1" },
 { wregex(L"f(f)"), L"$1" },
 { wregex(L"z(z)"), L"$1" },
+// There are a number of cases not covered by these rules.
+// Let's add some reasonable fallback rules.
+{ wregex(L"a"), L"â" },
+{ wregex(L"e"), L"@" },
+{ wregex(L"i"), L"ë" },
+{ wregex(L"o"), L"ö" },
+{ wregex(L"q"), L"k" },

src/recognition/g2pRules.rb

@@ -69,6 +69,17 @@ end
 # Read rules
 lines = File.read('../../lib/soundchange/english.sc', :encoding => 'iso-8859-1').split(/\r?\n/)
 
+# Add supplementary rules
+lines.push(
+  '* There are a number of cases not covered by these rules.',
+  '* Let\'s add some reasonable fallback rules.',
+  'a/â/_',
+  'e/@/_',
+  'i/ë/_',
+  'o/ö/_',
+  'q/k/_'
+)
+
 # Parse character class definitions
 characterClassLineCount = 0
 lines.each_with_index do |line, index|