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diff --git a/Day12.lean b/Day12.lean
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+import «Common»
+
+namespace Day12
+
+------------------------------------------------------------------------------------
+
+inductive SpringMask
+| operational
+| damaged
+| unknown
+
+structure SpringArrangement where
+  Mask : List SpringMask
+  DamagedGroups : List Nat
+
+inductive ParsingError
+| unknownSymbolInLeftBlock : Char → ParsingError
+| unknownSymbolInRightBlock : Char → ParsingError
+| missingBlockSeparator : ParsingError
+| noInput : ParsingError
+| missingLeftBlock : ParsingError
+| missingRightBlock : ParsingError
+
+instance : ToString ParsingError where
+  toString := λ
+  | .unknownSymbolInLeftBlock c => s!"Unkown character in left part of input: '{c}'. Expected '#', '.' or '?'."
+  | .unknownSymbolInRightBlock c => s!"Unknown character in right part of input. '{c}'. Expected either a numeric digit, or ','."
+  | .missingBlockSeparator => "An input line is missing a block separator (whitespace)."
+  | .noInput => "The input file is empty."
+  | .missingLeftBlock => "The left block, which should contain a map of operational ('.') or damaged ('#') springs, or the question mark ('?') is empty."
+  | .missingRightBlock => "The right block, which should contain a comma-separted list of numbers, is empty."
+
+
+
+def parse (input : String) : Except ParsingError SpringArrangement :=
+  sorry