Expression, new abstract base class for all WCCL expressions.

This is to accomodate rules in future, and repurpose "Operator" to be a library api-level class for users of the library, for parsed functional operators.

libwccl/ops/expression.h

+#ifndef LIBWCCL_OPS_EXPRESSION_H
+#define LIBWCCL_OPS_EXPRESSION_H
+
+#include <libcorpus2/tagset.h>
+#include <boost/noncopyable.hpp>
+
+namespace Wccl {
+
+/**
+ * Abstract base class for WCCL expressions
+ */
+class Expression : public boost::noncopyable {
+public:
+	/**
+	 * @returns String representation of the expression.
+	 */
+	virtual std::string to_string(const Corpus2::Tagset& tagset) const = 0;
+	/**
+	 * @returns String representation of the expression that does not
+	 * require a tagset.
+	 * @note Might be incomplete and/or contain internal info.
+	 */
+	virtual std::string to_raw_string() const = 0;
+};
+
+} /* end ns Wccl */
+
+#endif // LIBWCCL_OPS_EXPRESSION_H