code: def solve(a, b, sym_pos=None, assume_a="gen"):
      """
      Solve the equation ``a x = b`` for ``x``. By default, use LU
      decomposition and forward / backward substitutions. When ``assume_a = "pos"``
      use Cholesky decomposition.
  
      Parameters
      ----------
      a : (M, M) array_like
          A square matrix.
      b : (M,) or (M, N) array_like
          Right-hand side matrix in ``a x = b``.
      sym_pos : bool, optional
          Assume a is symmetric and positive definite. If ``True``, use Cholesky
          decomposition.
  
          .. note::
              ``sym_pos`` is deprecated and will be removed in a future version.
              Use ``assume_a = 'pos'`` instead.
  
      assume_a : {'gen', 'pos'}, optional
          Type of data matrix. It is used to choose the dedicated solver.
          Note that Dask does not support 'her' and 'sym' types.
  
          .. versionchanged:: 2022.8.0
              ``assume_a = 'pos'`` was previously defined as ``sym_pos = True``.
  
      Returns
      -------
      x : (M,) or (M, N) Array
          Solution to the system ``a x = b``.  Shape of the return matches the
          shape of `b`.
  
      See Also
      --------
      scipy.linalg.solve
      """
      if sym_pos is not None:
          warnings.warn(
              "The sym_pos keyword is deprecated and should be replaced by using ``assume_a = 'pos'``."
              "``sym_pos`` will be removed in a future version.",
              category=FutureWarning,
          )
          if sym_pos:
              assume_a = "pos"
  
      if assume_a == "pos":
          l, u = _cholesky(a)
      elif assume_a == "gen":
          p, l, u = lu(a)
          b = p.T.dot(b)
      else:
          raise ValueError(
              f"{assume_a = } is not a recognized matrix structure, "  # noqa: E251
              "valid structures in Dask are 'pos' and 'gen'."
          )
  
      uy = solve_triangular(l, b, lower=True)
      return solve_triangular(u, uy)
  
imports: from __future__ import annotations
  import operator
  import warnings
  from functools import partial
  from numbers import Number
  import numpy
  import tlz
  from dask.array.core import Array, concatenate, dotmany, from_delayed
  from dask.array.creation import eye
  from dask.array.random import RandomState, default_rng
  from dask.array.utils import array_safe, meta_from_array, solve_triangular_safe, svd_flip
  from dask.base import tokenize, wait
  from dask.blockwise import blockwise
  from dask.delayed import delayed
  from dask.highlevelgraph import HighLevelGraph
  from dask.utils import apply, derived_from
label_desc: Use image node_id 15 for calling a global function with example usage: solve(a, b, sym_pos, assume_a) and returns: solve_triangular