section{* The General Construction Principle \label{sec:ind-general-method} *}

  Before we start with the implementation, it is useful to describe the general

  form of inductive definitions that our package should accept. We closely follow

  the notation for inductive definitions introduced by Schwichtenberg

  \cite{Schwichtenberg-MLCF} for the Minlog system.

  Let $R_1,\ldots,R_n$ be mutually inductive predicates and $\vec{p}$ be

  parameters. Then the introduction rules for $R_1,\ldots,R_n$ may have

  the form

  \[

  \bigwedge\vec{x}_i.~\vec{A}_i \Longrightarrow \left(\bigwedge\vec{y}_{ij}.~\vec{B}_{ij} \Longrightarrow

  \qquad \mbox{for\ } i=1,\ldots,r

  \]

  where $\vec{A}_i$ and $\vec{B}_{ij}$ are formulae not containing $R_1,\ldots,R_n$.

  Note that by disallowing the inductive predicates to occur in $\vec{B}_{ij}$ we make sure

  that all occurrences of the predicates in the premises of the introduction rules are

  \emph{strictly positive}. This condition guarantees the existence of predicates

  that are closed under the introduction rules shown above. The inductive predicates

  $R_1,\ldots,R_n$ can then be defined as follows:

  \[

  \begin{array}{l@ {\qquad}l}

  P_{k_{ij}}~\vec{s}_{ij}\right)_{j=1,\ldots,m_i} \longrightarrow P_{l_i}~\vec{t}_i &

  \end{array}

  \]

  The (weak) induction rules for the inductive predicates $R_1,\ldots,R_n$ are

  \[

  \begin{array}{l@ {\qquad}l}

  P_{k_{ij}}~\vec{s}_{ij}\right)_{j=1,\ldots,m_i} \Longrightarrow P_{l_i}~\vec{t}_i &

  \end{array}

  \]

  Since $K_i$ and $I_i$ are equivalent modulo conversion between meta-level and object-level

  connectives, it is clear that the proof of the induction theorem is straightforward. We will

  therefore focus on the proof of the introduction rules. When proving the introduction rule

  shown above, we start by unfolding the definition of $R_1,\ldots,R_n$, which yields

  \[

  \bigwedge\vec{x}_i.~\vec{A}_i \Longrightarrow \left(\bigwedge\vec{y}_{ij}.~\vec{B}_{ij} \Longrightarrow

  \]

  where $\vec{K}$ abbreviates $K_1,\ldots,K_r$. Applying the introduction rules for

  $\forall$ and $\longrightarrow$ yields a proof state in which we have to prove $P_{l_i}~\vec{t}_i$

  from the additional assumptions $\vec{K}$. When using $K_{l_i}$ (converted to meta-logic format)

  to prove $P_{l_i}~\vec{t}_i$, we get subgoals $\vec{A}_i$ that are trivially solvable by assumption,

  as well as subgoals of the form

  \[

  \bigwedge\vec{y}_{ij}.~\vec{B}_{ij} \Longrightarrow P_{k_{ij}}~\vec{s}_{ij} \qquad \mbox{for\ } j=1,\ldots,m_i

  \]

  that can be solved using the assumptions

  \[

  \bigwedge\vec{y}_{ij}.~\vec{B}_{ij} \Longrightarrow

  \]